Reading about involuntary denied boarding / IDB, I notice that there is a US DOT rule about certain protected classes of passengers who must come last when IDB is necessary. The rule mentions minors and "Qualified Individuals with Disabilities:

2. Boarding Priorities - If a flight is Oversold, no one may be denied boarding against his/her will until UA or other carrier personnel first ask for volunteers who will give up their reservations willingly in exchange for compensation as determined by UA. If there are not enough volunteers, other Passengers may be denied boarding involuntarily in accordance with UA’s boarding priority:
a. Passengers who are Qualified Individuals with Disabilities, unaccompanied minors under the age of 18 years, or minors between the ages of 5 to 15 years who use the unaccompanied minor service, will be the last to be involuntarily denied boarding if it is determined by UA that such denial would constitute a hardship.

What constitutes a Qualified Individual with Disabilities? Is there a definition anywhere - in other words, how is it defined in IDB situations?

Thank you!