RFC 2474, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers," Internet Engineering Task Force (IETF), December 1998. (Defines Layer 3 DiffServ and the 6-bit DSCP field).
IEEE Std 802.1Q-2018, "IEEE Standard for Local and Metropolitan Area Networks—Bridges and Bridged Networks," IEEE Computer Society, Section 6.9.3 "Priority Code Point (PCP) encoding". (Defines Layer 2 classification using the 3-bit field for 8 priority levels).
Cisco Systems, "Quality of Service (QoS) Configuration Guide, Cisco IOS XE 17.x," Chapter: "QoS Overview," Section: "QoS Models." (Details Best Effort, IntServ, and DiffServ models).
Wi-Fi Alliance, "WMM (Wi-Fi Multimedia) Specification," Version 1.2.0. (Defines the implementation of IEEE 802.11e QoS and Access Categories).
Aruba (HPE), "ArubaOS 8.6.0.0 User Guide," Chapter: "Quality of Service," Section: "Wi-Fi Multimedia." (Explains WMM and its relation to 802.11e Access Categories).