Address of Record. It is the URI under which the endpoint registers, or, in practice, with Asterisk, the user part of it, and it is in the To: header of the REGISTER request.
In practice, it would normally the same as the endpoint name.
In practice, I think you would get that on a trunk, if the trunk was configured for outbound, or no, registration, and the endpoint tried to do an inbound registration.
In a register request, something that can be authenticated (From header), tells the registrar that the entity with its AOR can be contacted at the working address given in the Contact header. In typical FreePBX usage, the something that can be authenticated is the same as that which can be reached at the contact address, and its identity, for authentication purposes, is the same as its AOR, but this is a degenerate case, as far as the full generality of SIP is concerned.
Assuming that the link is configured statically, configure the TG100 to not attempt registration to the PBX. If you have already done that or there is no such setting, post details.