Each time a player joins your server, it creates a file which is the same name as the client, which shows :
The amount of mods they have
What the mods are
And also saves it each time they join, along with when they joined.
This mod is mainly used to prevent cheaters, but you can also use it if you dont want a certain mod being used on your server, but you can also use a mod whitelist on your server to make it so that you can make it so that players can only use mods that you choose, kinda useful I think