Each high stakes test has its own policies regarding what constitutes an impairment. To find out what documentation is needed prior to taking the test, you will need to go to that specific test website.
Below are the links to the website for each high stakes test and their disabilities accommodations webpage.
*As of May 20, 2014 - Big Changes in LSAT Accommodations for Applicants with Disabilities -UMassAmherst Pre-Law Advising Office (blog)
http://www.ada.org/sections/educationAndCareers/pdfs/dat_faq.pdf (Info at end of doc)
SW Licensing (ASWB—National test required for state licensure): http://www.aswb.org/SWLE/SWLE.asp
MI Bar Exam: http://www.michbar.org/professional/byexam.cfm
MI Teacher Certification: http://www.mttc.nesinc.com/