"The early Multiprotocol Label Switching (MPLS) documents defined the form of the MPLS label stack entry. This includes a three-bit field called the ""EXP field"". The exact use of this field was not defined by these documents@ except to state that it was to be ""reserved for experimental use"". Although the intended use of the EXP field was as a ""Class of Service"" (CoS) field@ it was not named a CoS field by these early documents because the use of such a CoS field was not considered to be sufficiently defined. Today a number of standards documents define its usage as a CoS field."