I study the InSim.txt and can't understand why there are so many different packets for text messages and commands.

What's the difference between IS_MSO, IS_MSX and IS_MTC?
What is IS_MSX for?
What does "messages in" and "messages out" mean? (I thought I got the logic, but now I'm confused)

MSO (message out) is for receiving chat messages from LFS.
MST (message in) is for putting chat messages or commands into LFS.

MSX is the same as MST, but longer for chat.
MSL only appears locally (basically the same as an MST or MSX with the /echo command).

MTC is for sending a message to a specific connection only.

Thanks.