It is Type of Service and implemented as IPv4 TOS priority control. It is a fully decoded to determine the priority from the 6 bit TOS field in the IP header. The most significant 6 bits of the TOS field are fully decoded into 64 possibilities, and the singular code that results is compared against the corresponding bit in the IPv4 TOS priority control bit(0~63). If the bit is set, the priority is high; otherwise, the priority is low.
Three bits: IP priority (0 to 7)

One bit: No delay
One bit: high throughput
One bit: high reliability
Two bits: reserved