IP Header is defined in RFC 791, this header includes a 1 byte field called the Type of Service (ToS) byte. This Tos byte is used to mark the packet for a specific type of treatment or prioritization.
The ToS byte itself is subdivided with the high order 3 bits defined as the IP Precedence field (IPP)
The use of the high order 3 bits IPP in the ToS byte determined prioritization for the packet, later a series of RFCs called Differentiated Services (DiffServ) came along, which required redefining the use of the ToS byte because DiffServ required more than 3 bits. The ToS byte was renamed the Differentiated Services Field and IPP was replaced with a 6 bit field called Differentiated Services Code Point (DSCP).
Later RFC 3168 defined a use for the last two low order bits in the DiffServ field for use with the QoS Explicit Congestion Notification Feature.