The server is in charge of generating it. There's a packet the server can use to change it, and the client sends it to the server on every login.
I think it's probably a random number but given the server generates it it is tricky to be sure (https://dilbert.com/strip/2001-10-25).
The file contains exactly 24 bytes, you could read it with a hex editor (a normal text editor probably won't be happy because it's binary).