Something I noticed when playing around with the copy command and which I briefly mentioned in the Vertical Backup issues section…
The copy command doesn’t actually seem to support tags.
Looking through the other commands, nor does the history command, although I suppose it’s a bit less useful.
But it makes sense that any command that supports specifying more than two revisions or revision ranges, that it should let you use tags instead. And I can envisage tags being very useful when combined with copying between tiered storages.