Storage Replicator Node
The Storage Replicator and the Verifier are the same node with different roles. This node stores the Storage Director's data (and gets paid for this), and also verifies other Replicators.
Replicator Responsibilities
Replicator nodes search for and join active Drives. When a Replicator wants to join a Drive, that Replicator makes a deposit equal to the size of the Drive.
Replicator nodes store the Storage Director's files. When new data should be stored, the Storage Director sends this data to the Replicator nodes, and they store it until the Drive's contract is expired or the Drive itself is closed.
Replicator nodes confirm that they actually store the files. Verifiers regularly check that the Replicators store correct and undamaged files. To do this, Verifiers use the Challenge. Verified Replicator nodes, called Provers, must generate and send a response according to the verification protocol that is formed using the stored data. Replicators that fail the verification are removed from the Drive.
Verifier Responsibilities
The Verifier is a node that checks that data stored by Storage Replicator nodes is untouched and safe. It uses the Challenge to control Replicators and to prevent corruption and loss of the Storage Director's data. The Verifier begins verifications automatically on specific blocks, depending on the hash of the block, the key of the Drive, and a special constant that is set in the network configuration and which regulates the frequency of the verifications.
According to the verification algorithm, a Verifier that fails verification loses its deposit. That deposit will be divided between Replicators of that Drive. Since the main goal of any Storage Replicator node is to make profit, it is interested in verifications.