> The time is measured in units of seconds, but since every module that processes a datagram must decrease the TTL by at least one even if it process the datagram in less than a second, the TTL must be thought of only as an upper bound on the time a datagram may exist.
It just says that outputTTL is (inputTTL - 1). With some exceptions.
[edit: I missed that that RFC is for MPLS but would be interested in your comment anyway; the definitive version seems to be https://datatracker.ietf.org/doc/html/rfc1122]