From a recent code review for SERVER-77348:

The way I see it, there should be only the following ways to ask for a namespace and either it should be just 1 or all, but not some combination of the three:

Either I give you the namespace (db.coll) and I don't care what's behind, which should be 99% of the cases
OR I give you the DB+UUID and I don't care what's the name
OR I give you all of the above and I want them all to match

There are a few places where we track an optional nss, dbName, and uuid, which look deceptively similar to a NamespaceStringOrUUID. We should consider introducing a new type to express the case that NamespaceStringOrUUID cannot currently handle: NamespaceString and UUID. Another choice might be to support a third state in the NamespaceStringOrUUID union (and perhaps rename that type to something like CollectionOrViewLocator).

Examples:

• https://github.com/mongodb/mongo/blob/a1776917bebc14c1ba822480c63f55d3af7b2804/src/mongo/db/shard_role.h#L91-L94
• https://github.com/mongodb/mongo/blob/aaf8464c54244781177f57f341f59a324bac39ca/src/mongo/db/pipeline/expression_context.h#L470-L475
• https://github.com/mongodb/mongo/blob/aaf8464c54244781177f57f341f59a324bac39ca/src/mongo/db/catalog/uncommitted_catalog_updates.h#L351-L352