This would be an application-wide or Location-wide setting. The most obvious use would be with, say, an Archive tag. Documents that are no longer relevant could be tagged with Archive, and then wouldn't show up in any searches unless "+Archive" was explicitly included in the query.

This could also be useful for images that are intended solely or primarily for inclusion into MD or HTML documents. Right now, I'm using the tag Src and no other tag to label images that I don't intend to locate in a search, but that are embedded in other documents. It would be nice to add other tags to those images so that searching through them would be easier when I intend to do so, but I cannot do that as things are because this would cause them to appear in other searches in which I'm looking for primary files only, and not their related embedded media.

On the topic of tagging image files that are embedded in other documents, the fact that the filename is used as the tag store is a problem, as indicated here: https://tagspaces.uservoice.com/forums/213931-general/suggestions/5789542-save-tag-data-and-metadata-in-a-separate-file