distinction between primary and secondary usergroup

[Hornstar]

I have a client that is making a script that will be uploaded to my server that will need to do several things. I'd like to use this thread to talk about some of those things and to help him in a few of his questions, but overall once its finished I wanted to ask everyone if what he has made will be safe and secure to use for the purpose it has. (Will go into more details on that later.

first, he wanted to know how vbulletin makes the distinction between primary and secondary usergroups.

Thanks for any help that the coders can provide.