Member Categories lets you link custom member fields tocategory groups. It's a simple concept - but also a very powerful one. If you think about it a category group is just a list ... it can be a list of member groups, a list of teams or any other kind of category you'd like to put your members in.
You can link as many member fields to as many category groups you want. For each linked field, members can be put in either one or multiple categories.