Need help with VATSIM

I have been flying IVAO for a few years, and I want to switch over to VATSIM. I've been reading there pilot resources guides, but I cant seem to understand what they use for MTL modeling? I've downloaded squawk btw.
Flight Simulator and other chat
https://forums.simviation.com/phpBB3/
https://forums.simviation.com/phpBB3/viewtopic.php?f=52&t=164213