The source DESCRIPTION for this object is an empty string, so its exact intended semantics aren't documented in the MIB itself, but based on its name, its DEFVAL of "none", and its position (indexed by ifIndex) alongside failover/failback timers and a status flag in a table named for Dial Backup, it appears to name the primary WAN interface that a given dial-backup configuration entry is monitoring or backing up. A network administrator would set this to identify which primary link a backup dial connection should activate for when it fails. For example, setting this to an interface name like "serial0" would designate that serial WAN link as the one this dial-backup entry watches over.