In a Registered Trunk connection, a username and password are required to register a SIP Client to the PBX and then receive and initiate calls via SIP Invites to/from the PBX in the same manner as a SIP client.
In a Direct IP Trunk or non-registered connection, only the user name is necessary to receive and initiate calls via the PBX. There is no registration or authentication, only SIP Invites to/from the PBX. At some installations, this method is used to install a SIP Trunk between the VLink TIF server and an offsite 3rd Party provider for the phone line access.
Note, if using option #2 be sure to set Disable Domain Authentication to On.