Under the Americans with Disabilities Act, people with service dogs are not required to register their animals or to carry any documentation with them. Asking for documentation is illegal. However, people with Emotional Support Animals are required to carry a letter from a licensed medical provider.
Service dogs and Emotional Support Animals differ in that service animals are trained to perform tasks for their owners, while Emotional Support Animals are not trained. Although Emotional Support Animals are associated with owners who have mental disabilities, especially anxiety disorders, the difference between them does not lie in mental versus physical disability; some service dogs are trained in calming people with anxiety disorders such as PTSD. Federal law mandates that Emotional Support Animals must be documented, to distinguish them from ordinary pets. Service dogs owners do not need to register their animals, although national registries do exist to protect owners from discrimination.