I created a separate group just so they are in one place for maintenance/upkeep/clean organization purposes. But, I don't know if it is necessary to add a specific group for Radio IDs - you probably could add them in any of the groups you have.
I'm taking an educated guess here: Radio ID information is part of the data stream whenever there is a channel grant, just like talkgroup ID and specific voice frequency. The scanner's logic process will "grab" that Radio ID data, look to see if it is in the programming and if so, display the alpha tag associated with the Radio ID (including alert light and tone).
It's sort of like when ID Searching. If the scanner "hears" a talkgroup, it looks to see if is programmed and if so, does it have an alpha tag. If there is an alpha tag, then the alpha tag is displayed - if not, just the talkgroup ID number is displayed.
On Radio IDs, on our system Radio ID number 180300 is assigned to the mobile radio for Engine Company 8030. I have i180300 programmed as a radio ID (in the talkgroup column) and "Engine 8030-M" as the alpha tag. When that radio keys up, I'll see Engine 8030-M on the scanner display. If I did not have that programmed, then all I would see is "UID:i180300".
Hopefully I won't confuse you with the following. When "doing it's thing" scanners are only "scanning" the control channel for a trunked system. They are not "scanning" the groups which have talkgroup IDs or radio IDs, in the sense of looking for one, then another, then another, etc.
These groups are lists of TGIDs or Radio IDs are lists of "stuff" you might wish to hear.
When that channel grant happens, the scanner "notes" the talkgroup ID and Radio ID contained in the data stream. The scanner then looks into your lists of things. If it finds the Talkgroup ID and Radio ID and they haven't been locked out, the scanner sets itself to the voice channel, turns on the audio, and displays the alpha tags (along with alerting stuff).
Suffice it to say monitoring a trunked radio system is a whole 'nother ballgame when compared to rolling through a group of conventional radio frequencies.