Selecting tags in the tag view filter the list into items that contain all of the tags selected.
It would be really nice if, upon selecting a tag, the tag view (perhaps optionally) filter in the following way: first tag selected (say “A”) ==> list view updates; tag view now shows, in addition to “A” selected, only those tags that are associated with the items that are still in the list view (i.e., have tag “A”).
DEVONthink can work with tags both ways. If you select multiple tags in the Tags view, then the list of documents is subtractive (documents returned contain All of the selected tags). When you select multiple tags in other views, say the Three Pane view, then the list of documents is additive (documents returned contain Any of the selected tags).