This applies to people running a business (for the most part), but keep in mind that it doesn't apply everywhere. The FHA has its own set of rules for both service and emotional support animals, and it is somewhat more lenient than the ADA. (This applies primarily to housing, what a landlord is or isn't required to allow).

https://www.ada.gov/regs2010/service_animal_qa.html

Also, don't be fooled by 'certificates' for support animals, you can buy one on the web for a pet rock if you so desire. They hold no weight whatsoever as far as the law is concerned. And a valid service animal isn't required to be certified anyway (as far as the FHA is concerned anyway).