This is the MPLS label this device pushes onto packets entering this dynamically-signaled tunnel, negotiated automatically via RSVP-TE label distribution rather than manually assigned. An admin trusts this value to always match what the next hop expects, since the signaling protocol itself, not the operator, is responsible for keeping both ends synchronized -- a persistent mismatch here would point to a signaling protocol bug or version incompatibility rather than operator error. For example, this label reading 400 after a successful RESV exchange confirms RSVP-TE completed label negotiation correctly for this specific LSP.