could you explain why you are using single purpose in relation to pet?
often a single purpose dog may be a dog that has a redirection in career . For example a dog that has passed all the requirements for nerve, drive, orthopedics, health, etc etc. but the dog may be too friendly , fail to engage a "bad guy" , fight drive , aggression just not there . Now we are talking about GSD that are supposed to be able to control a situation , to hold , and hold by bite if necessary.
This dog may have a career as a specialized detection dog.
Some categories are so demanding and requiring so much accuracy and precision (bomb/explosive) that these are single purpose , no confusion .
A golden retriever would not have this requirement as this is not part of the breed profile , nor an expectation.
Some dogs are very very effective as search and apprehend , or SWAT , others have talent enough to train and certify them as competent in detection and apprehension , so they are "dual" .
Has someone tried to promote something to you. Tell more about your situation.
Carmspack Working German Shepherd Dogs