The first thing that happens is that the radio requests registration on the system, i.e. "login". The radio broadcasts its Subscriber ID along with the SysID, the system verifies that it's a valid ID, and the radio is allowed to register.
The next thing that happens is that the radio requests affiliation with whichever talkgroup is selected. The system verifies that the talkgroup is valid for the system and (in the case of multi-site systems) at that particular site, and the radio is allowed to affiliate.
It's takes a fraction of a second for both steps to occur.
14:07:43.606 842 -AC 00 08 40 03 5B C6 03 5B C6 97 01 Unit Registration Response Sys 840 ID 035BC6 WUID 035BC6 accept
14:07:43.859 842 28 00 00 00 00 55 F1 03 5B C6 61 BA Group Affiliation Response I 035BC6 accept 55F1