Tags and groups are rather similar in DEVONthink, although there are important differences. Indeed, if the option to Exclude groups from tagging in File > Database Properties for a database is unchecked, groups will be treated as tags. However in the Tags view (View > as Tags) “group tags” will be displayed in a different color than “tag tags”.
The big difference is that groups can be used as the location into which documents are filed, whereas “tag” tags don’t “contain” the documents (assuming proper techniques for tagging are followed).
In the discussion below, I’m assuming that we are talking about “tag tags” and not “group tags”.
When you tag a document by using the Tags bar in a view, or by dragging one or more selected documents into the Tags area of the Groups & Tags panel, or by using the Tag area of a HUD panel you are not copying the document into the tag. Instead, you are adding a few bits of information to that tag, which will thereafter retain the tag information in DEVONthink. This requires very little data overhead.
Once a tag has been added to a document, the tag can be deleted by selecting and deleting the assigned tag in the Tags bar, or by clicking on the tag, selecting the document to have the tag removed from the list displayed and choosing Delete, &c.
But if a document is selected within the list displayed when a tag is selected and dragged into a group a replicant of the document will be created (and the tag will be removed). If the item is dragged into the group in which the document is actually located, that group will then contain two replicants representing the document. (in the case of replicants, there’s still only one underlying file, and the database overhead to display each replicant is only a few bits.)
That’s why I was puzzled by the action to do that in a post above. Why? It’s a convoluted procedure to produce untagged replicants in the same or different groups in a database. There are more straightforward ways to replicate items.
If adding a tag to a document was desired in the first place, why then move the item out of the tag to a group location? The tag will be lost, and replicants will be created.
If the decision is made to remove a tag after initially assigning it, don’t drag the item from the tag to a group (unless replication is desired). Instead, delete the tag in the document’s Tag bar, or delete the document from the list displayed when a tag is selected. If an existing tag is no longer wanted, delete the tag itself, and all documents previously tagged by that tag will no long be so tagged.