This is the MPLS label this device expects to see on packets arriving back on the reverse leg of this co-routed tunnel -- the counterpart to the forward-out-label used for the outbound direction. An admin checks this against what the far-end device is actually configured to push for the return path, since a mismatch here specifically breaks inbound/return traffic while outbound traffic (governed by the separate forward-out-label) keeps working fine. For example, this device expecting reverse-in label 301 while the far end actually pushes 305 for the return path would explain why a customer reports 'I can send but responses never come back' on this specific circuit.