Subagency is left over from the dinosaur era. When ALL agents represent the Seller. We offer compensation to transaction brokers and buyer agents. So the agent bringing the buyer will get paid whether they represent the buyer or not. There is no need to allow the agent bringing the buyer to represent the Seller. If you explain subagency to a Seller clearly, and I have, they will demand that you not permit this.
The real question should be: Why on earth would you permit and compensate subagency?
That being said, there is no reason why you couldn't work with another agency to facilitate showings of a property, for example, and figure out how to compensate them without entering into a sub-agency agreement. That's done quite frequently as long as the seller approves. However, if they do show the house, it must be clear that they are in no way affiliated with your company or acting on the seller's behalf.
I think it is very important for the client to know who has the fiduciary responsibility to them in addition to the other responsibilities that go along with a client/agency relationship and not to muddy the waters with a sub-agent.
The relationship form (form 3) does list 3 types of relationships the person can have with the broker, but they are single agency, appointed agency, and disclosed dual agent. Otherwise you are a customer. There is no mention of subagency on this form.