Reference: Decrypt_Files
This field is optional when decrypting. When included, it provides a consistency check on the descriptors used to encrypt the archive. The key name is not important. The only issue for encryption and decryption is that the pass phrases used to create the keys are exact matches.
Warning: Do not use a key that is also being used for Beyond FTP server communications! These keys may be updated and changed in ways you do not expect. ALWAYS USE A KEY WITH A KNOWN PASS PHRASE! This will prevent key retirement or secondary key demotion from making your archive inaccessible!