Attributes are a transaction type added to Indy to allow a DID owner to add data to a DID.  It is used by convention to add an endpoint to a DID for DIDComm.  When a DID is resolved, the DIDDoc mostly comes from the DID, but if there is an associated ATTRIB with an endpoint, it is added to the DIDDoc as a service endpoint. (To be changed in did:indy).