Part one of this memorandum contains a summary of questions asked in the DOL request for information about the implications of providing accommodations to employees with serious health conditions to enable them to perform their jobs with accommodations, rather than taking FLMA leave.
Part two of this memorandum contains the relevant statutory and regulatory text. Part two also lists other sources cited in the comments about this topic.
Workplace Flexibility 2010, Georgetown University Law Center, "Essential Functions" (2007). Memos and Fact Sheets. 40.