Almost every hard employment question eventually routes through one document: the job description. Can this employee do the job with a restriction? Is remote work required? Was the person who was let go actually qualified? The answer turns on what the job's essential functions are — and the written description is the first place anyone looks.
The Americans with Disabilities Act builds this in explicitly. Its definition of a qualified individual asks whether the person can perform the essential functions of the position with or without reasonable accommodation, and it instructs that consideration shall be given to the employer's judgment. Crucially, 42 U.S.C. § 12111(8) provides that if an employer has prepared a written description before advertising or interviewing for the job, that description is evidence of the essential functions. Written after the dispute starts, it is just argument.
Essential versus marginal, in practice
An essential function is a fundamental duty of the position — a reason the job exists. A marginal function is something the person happens to do that could be reassigned without changing the nature of the role. The distinction decides real cases, because an accommodation that removes an essential function is generally not required, while shifting a marginal one often is.
The EEOC and courts look at a cluster of practical indicators rather than one test.
| Signal | Points toward essential | Points toward marginal |
|---|---|---|
| Why the position exists | The duty is the purpose of the role (a driver drives) | The duty supports the role but is incidental |
| Time spent | A substantial share of the workday or cycle | Occasional, seasonal, or a few minutes a week |
| Consequence of not performing | Safety, revenue, or service failure | Minor inconvenience absorbed elsewhere |
| Number of employees available | Few people can do it; redistribution is impractical | Several colleagues already perform it interchangeably |
| Specialization required | Position was filled for that specific expertise | Any team member could pick it up |
| Documentation | Listed as essential in a pre-advertisement description and in past postings | Absent from the description or added late |
Two categories generate the most argument. Attendance and schedule requirements are often essential — but "regular attendance" stated as a bare slogan is weaker than a description explaining why coverage during specific hours matters. Physical requirements copied from a generic template are the other trap: a warehouse description that recites "must lift 50 pounds" when the heaviest routine item weighs 20 invites a challenge to every restriction-based decision that follows.
Writing a description that holds up
The useful version is short, specific, and observably true. It describes outcomes and conditions rather than habits, and it separates the duties that define the job from those that merely fill the day.
- Start with the incumbent's real week. Ask the person doing the job and their supervisor to log tasks for a representative period. Templates describe an imagined job; logs describe the one you have.
- Sort every duty into essential or marginal. For each, answer why the role exists, what happens if the task goes undone, and who else could do it. Write the reasoning down; you will want it later.
- State functions as results, not methods. "Move product from receiving to shelving" leaves room for a cart or a lift. "Carry boxes by hand" needlessly builds a method into the standard.
- Quantify only what you can verify. Weights, distances, hours, and frequencies should reflect measurements, not guesses. An unverifiable number is worse than none.
- Name the working conditions honestly. Noise, temperature, driving, on-call rotation, travel percentage, and safety-sensitive designations belong in the document if they are real.
- Date it and file it before you advertise. That sequence is what converts the description into the evidence § 12111(8) contemplates.
Watch out: a description crowded with every conceivable duty marked "essential" is a liability, not a shield. It makes the classification look indiscriminate, and it hands an employee a list of things the employer must now prove actually matter. Over-inclusion also collides with hiring: a requirement no one truly needs can screen out qualified applicants and become the basis of a disparate-impact claim.
Where the description shows up later
Accommodation is the obvious use. When a request arrives, the description frames the analysis: which functions are essential, which of them the limitation affects, and what changes would let the person perform them. That analysis is the spine of the interactive accommodation process, and a stale description makes the whole conversation guesswork. The free Job Accommodation Network catalogues accommodation options by job task, which is far easier to use when tasks are written plainly.
Medical certifications are a second use. Providers asked "can this patient work?" give vague answers; providers given a functional description give usable ones. That matters when leave and accommodation run together, as mapped in our guide to overlapping FMLA, ADA, and state leave rights.
Performance decisions are the third. A supervisor who can point to the written function a person failed to perform is in a different position from one who says the employee "wasn't a fit" — the reason performance documentation and job descriptions should be written to reference each other.
Finally, descriptions drive pay structure. Job architecture — how roles are grouped and levelled — is the foundation of any credible pay equity review, because comparing pay across roles requires first knowing which roles are comparable.
Keeping them current without a permanent project
Descriptions rot quietly. Software changes, a location closes, a duty migrates to another team, and nobody edits the file. A workable cadence: refresh a description whenever the role is posted, whenever an incumbent changes, and once a year for safety-sensitive or physically demanding jobs. Keep superseded versions with their dates — the version in effect at the time of a decision is the one that will be examined.
- Dated, version-controlled description files with the pre-posting date visible
- The posting or advertisement that used that version
- Task logs or supervisor notes supporting the essential/marginal calls
- Measurements behind any physical or schedule requirement
- Records showing the same description was applied to everyone in the role
- Notes of any accommodation that modified how a function was performed, and whether it worked
Quick answers
If a duty is in the written description, is it automatically essential?
No. The written description is evidence, not a conclusion. A duty listed but never performed, or performed by anyone on the team without difficulty, can be found marginal despite the label. Conversely, a genuinely fundamental duty left off the page can still be essential. The document is persuasive because it usually reflects reality — when it does not, reality wins.
Can we require a doctor to clear an employee for every listed function?
You may share the functional description and ask whether the employee can perform the essential functions, with or without accommodation. What you cannot do is treat a blanket "not cleared" as the end of the inquiry, or demand full medical records. If the provider flags a limitation, the duty is to explore accommodations for that specific function, not to remove the person from the job automatically.
Do we need a written description at all?
No federal statute requires one. But without it the employer forfeits the evidentiary advantage the ADA offers, loses the anchor for medical certifications and performance standards, and has a harder time defending screening criteria. For federal contractors and for regulated safety-sensitive roles, written descriptions are effectively expected as a recordkeeping matter.
Does listing "must be able to work overtime" protect us?
Only if it is true and applied consistently. If overtime is genuinely required by the operation, say so with specifics — typical frequency, notice given, coverage consequences. If it is aspirational or applied selectively, the clause does little and can undercut the credibility of the rest of the document.
Who should approve the final text?
The supervisor who observes the work should draft it, HR should check it against the classification and pay structure, and someone should review physical, schedule, and safety language against actual measurements. For roles where restrictions are common, having employment counsel review the template once is usually enough to fix recurring drafting problems across the whole library.
Your next moves
Pick the five roles where accommodation requests, medical restrictions, or turnover disputes actually arise, and rewrite those descriptions first from real task logs. Date them, file them before the next posting goes out, and store the reasoning behind each essential-function call alongside the document. Then connect them: reference the same function language in performance standards, in medical certification requests, and in the handbook's description of how to ask for a change — the approach described in our piece on handbooks that match actual practice. Related employer duties across accommodation, leave, and discipline sit together in the Workplace Rights & HR pathway. General ADA background for employers is published at ADA.gov.