The US Department of Justice (DOJ) oversees a Civil Rights Division and compliance relating to the Americans with Disabilities Act (ADA). The ADA is a federal law that establishes a number of rights and guarantees of equal access for US citizens with mental or physical disabilities.
The DOJ is therefore in charge of enforcing the federal regulations that protect the rights of individuals with disabilities, including those who are partnered with a hearing, guide, or service dog.
As an essential aspect of this, the agency establishes the legal definition of an assistance dog. This officially identifies these types of working animals, and also differentiates them from other animals they’re often confused with, such as emotional support and therapy animals (particularly because people using these latter types of animals do not have legally protected equal access under the ADA).
The web page linked below provides important official information about how assistance dogs are defined under the law. It establishes the broad definition that an assistance animal is a dog individually trained to perform work or tasks to mitigate a person’s physical or mental disability, and then goes on to elaborate in more detail.
This section of the DOJ website also offers an overview of access rights of people partnered with an assistance dog, as well as their responsibilities when taking the dog into places of business or other public areas. And it provides some important information for people who operate or work in business establishments or other publicly accessible areas. This includes information about how such people may seek to determine whether a particular dog is a legally protected assistance animal.
In general, ADA.gov is an excellent go-to resource for information about the Americans with Disabilities Act, the rights of people with disabilities, accommodating people with disabilities, and assistance dogs.