Users in the forum may also belong to multiple user groups. This is one method of creating exceptions to your permission scheme.

For example, you may have a user who belongs to group X, but needs to have access to the few extra options (such as attaching files to posts) given by group Y. You can make the user a member of both user group X and user group Y.

If a user is in multiple groups and they specify conflicting permissions, the greater permission overrides. Thus, a yes will always override a no.