The "verifier id" is a tagged hash of the encrypted file to be uploaded.

It is used as an index to the StorageServer? service on each peer. Each StorageServer? holds zero or more shares for each file in objects called "buckets": the collection of these buckets is indexed by verifierid.

The same plaintext file can be encrypted in multiple ways (by changing the encryption key): each different resulting crypttext will have a different verifierid. As a result there is a one-to-many mapping from FileId to VerifierId.