I don't use the Flag marker so I can use it for subset. If I want a subset of just face "Alton" and nobody else I select a face that is in "Alton" pictures, Use Ctrl A to select all, then Photo/Mark as/Flag. I flag all the faces that I want to NOT be in the final subset. Then I pick Face "Alton" and set the filter to "Flagged". Now my subset contains pictures with Alton only. This same method can reach across types. You can flag a Tag then pick an Album and filter out the flagged pictures. Just remember to reset the flag off when you are through.