MTLib is a library mod for use by Crafttweaker addons, it does not have to be used, but it does provide some helpful methods and classes to make Crafttweaker addons easier to create.
This includes all of Modtweaker util / helper classes, such as the IUndoable action classes and base command "dump" classes.
If you would like to support me in my modding endeavors, you can become a patron via 
Patreon.
This project is sponsored by Nodecraft. Use code JARED for 30% off your first month of service!