This is the filename of the certificate to be imported for the configuration for fetching and importing a trust certificate used to secure the VMM connection. It's the specific file UCS Manager will fetch from the remote path defined by the keyring's location and path fields, so a typo here is a common reason a certificate import for that keyring fails to find anything. A value like 'vcenter-ca.pem' would tell the import job to fetch exactly that file from the configured server.