Super Colliding Nix Stores

I don’t understand from “Lower metadata source:” on the linked documentation how the metadata source is configured. It says:

This is abstract, just some way to read the metadata of lower store store objects. For example it could be a SQLite database […]

It’s not clear from these docs what the options are for providing the lower metadata?

It also says of “Lower store directory”:

Specified with lower-store.real setting.

However I didn’t find any examples of this being used?