A companion dog is typically a dog who makes us feel better or calms our nerves. These dogs give us joy and love when we are low. Some companion dogs can be trained to become emotional support dogs however, an important difference is an emotional support dog that will be accompanying its handler into public places must be behaviorally sound and in no way display any sorts of anxiety, aggression or hyperactivity.
Any dog that goes into a public place must be calm. It should be focused on its handler and not on its surroundings. Any breed can be an excellent companion dog in your own home, even if it does not display the proper behavior for public access. There is training available (UKP/MK) if you have a dog you believe can support you as an emotional support dog or service dog and you have a documented condition that could be mitigated with a service dog.