PID = port ID. In STP (Spanning Tree Protocol), if the BID in the BPDU received by the port is the same as the path cost, the PID is compared to select the blocked port. The term PID (packet identifier) in digital TV multiplexing system is like the file name of a file, which we can call "identification code transmission packet". PID (Proportional Integral Differential) in engineering control and mathematical physics is a mathematical physics term. PID is composed of 8-bit port priority plus port number, the port number is low, and the default port number priority is 128.