sorry if I was too brief
I try to explain with a short example:
there are 3 tags (or categories): t1, t2, t3 and 3 posts post1 (t1,t2); post2 (t1,t3), post3(t3)
in wp show is possible to point more choices (example t1,t3) but in viewing all posts will be visible not only post2 that is t1 and t3 (type boolean logic).
In any case your plugin is very useful and I recommend it.