29 CFR § 825.123 - Unable to perform the functions of the position.
(a) Definition. An employee is unable to perform the functions of the position where the health care provider finds that the employee is unable to work at all or is unable to perform any one of the essential functions of the employee's position within the meaning of the Americans with Disabilities Act (ADA), as amended, 42 U.S.C. 12101 et seq., and the regulations at 29 CFR 1630.2(n). An employee who must be absent from work to receive medical treatment for a serious health condition is considered to be unable to perform the essential functions of the position during the absence for treatment.
(b) Statement of functions. An employer has the option, in requiring certification from a health care provider, to provide a statement of the essential functions of the employee's position for the health care provider to review. A sufficient medical certification must specify what functions of the employee's position the employee is unable to perform so that the employer can then determine whether the employee is unable to perform one or more essential functions of the employee's position. For purposes of FMLA, the essential functions of the employee's position are to be determined with reference to the position the employee held at the time notice is given or leave commenced, whichever is earlier. See § 825.306.