IP Precedence Values 6 and 7 Reserved for Control Plane traffic
The Type of Service (ToS) byte was defined in RFC 791 way back in 1981 when IPv4 was also defined. The ToS byte as part of the IPv4 header and looks like this:
The values of IP precedence ranging from 0 to 7 are defined in the 3-bit precedence field, and they have very specific purposes. Values 6 (110) and 7 (111) are reserved for Network Internetwork Control and Network Control respectively. According to the RFC, Value 6 is known as the network control precedence designation, while a value of 7 is known as the internetwork control designation. According to the RFC:
The Network Control precedence designation is intended to be used within a network only. The actual use and control of that designation is up to each network. The Internetwork Control designation is intended for use by gateway control originators only. If the actual use of these precedence designations is of concern to a particular network, it is the responsibility of that network to control the access to, and use of, those precedence designations.
The precedence values defined here are simply an expression of how they are intended to be used. Although they say they are "reserved" in truth, the reservation is only a guideline.
What they end up being used for is completely up to the network administrator.
This reservation should not be confused with the reservation of ToS bits 6 and 7.
Links
https://networklessons.com/quality-of-service/ip-precedence-dscp-values#Class-Selector_PHB