Skills routing and ACD · foundation · 12 min full read
The same chapter says users can be assigned to multiple skill groups, and campaigns can have multiple skill groups. Membership is not a single label for life.
A campaign with several groups is a pool you chose. A user in several groups can be offered work from more than one queue. This page does not say which queue wins when both are ringing. That tie-break did not come back in this check.