In the docs the following is mentioned:
-id <snapshot id> delete revisions with the specified snapshot ID instead of the default one
What exactly is “the default one”?
Technically, the default ID is the first entry in the preference file.
Usually this has a name of default when you don’t specify a name during init, but the first entry is always used when no ID is specified in operations such as prune.
Bare in mind, though, prune is typically used with -all instead - to prune all the snapshots in the storage.