http://www.rfc-editor.org/rfc/rfc5462.txt
RFC 5462: Multiprotocol Label Switching (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic Class" Field, L. Andersson, R. Asati2009-02-01T23:00:00-00:00To avoid misunderstanding about how this field may be used, it has become increasingly necessary to rename this field. This document changes the name of the field to the "Traffic Class field" ("TC field"). In doing so, it also updates documents that define the current use of the EXP field. [STANDARDS-TRACK]