Service dog at work at an airport
A service dog's work is to assist a person with a physical or mental disability. The Americans with Disabilities ACT (ADA) is the federal law that defines service dogs. The definition is as follows:
Service animals are defined as dogs that are individually trained to do work or perform tasks for people with disabilities. Examples of such work or tasks include guiding people who are blind, alerting people who are deaf, pulling a wheelchair, alerting and protecting a person who is having a seizure, reminding a person with mental illness to take prescribed medications, calming a person with Post Traumatic Stress Disorder (PTSD) during an anxiety attack, or performing other duties.
Service dogs are allowed in almost all settings where the public is allowed. Because of their intelligence, Havanese are good candidates for some types of service work. While they're not large enough to pull a wheelchair, or to provide physical support to a person with a mobility disability, they may excel at determining when blood sugar is out of control, or when certain types of vital medications are needed, or recognizing when a seizure is coming on, or alerting their deaf human when the doorbell rings, a smoke alarm goes off, etc.
Note that "emotional support animals" are NOT considered service animals under the ADA.
Challenges for the Dog
The dog should be able to adapt to stressful situations and a variety of unfamiliar surroundings. The dog needs to be able to on duty any time the handler needs the support.
Challenges for the Handler
The handler must be able to manage the dog's care, either him/herself or arrange with an other person, such as feeding, elimination, grooming, health care, etc. The handler or other person should be able to recognize when the dog is not well, overly tired, etc.
The dog must be trained for the handler/owner's specific needs. This does not require a professional trainer, if the handler/owner has other resources for the training. The dog should have basic manners, such as from CGC type training. If a service dog is not under control, including on leash, harness, etc., a business has the right to ask that the dog be removed from the premises.