The IUser interface is used internally to combine different userTypes, like the minecraft console, a normal user and command blocks in one type.
You will probably seldomly need this.

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import.
import crafttweaker.player.IUser;

IUser extends ICommandSender. That means all functions available to ICommandSender Objects also are available to IUser Objects.

Currently there are no distinct functions for IUser objects.