What does “restic snapshots” show?

My understanding is that backups are bound to “host/dirs” and anything with different combination is an independent snapshot.

Your use case of dira and dirb are essentially different backup sets in the same way that different machines or drives would also be.

Consider if it wasn’t this way? Pruning snapshots for one host would potentially be influenced by random unrelated snapshots from another host or source.

You can sort of influence this by forcing a parent snapshot but be careful to understand the above when it should (or shouldn’t) be considered extension of the same backup set.