Selects which value the DHCP relay agent inserts as the Circuit ID in DHCP option 82: the logical interface (li), the physical port, a configured CID string, or the LI-plus-VS combination (liVs). The circuit ID is what the DHCP server uses to identify exactly which access-side interface a lease request came from, so this setting determines how granular or human-meaningful that identification is when the server (or an admin reading server logs) tries to trace a lease back to a physical port. Selecting cidString(3) means the relay agent stamps a pre-configured string like 'Building3-Floor2-Port14' into option 82 instead of an opaque numeric interface identifier.