What's the difference between the level contols for main and the output level control?
What do you mean? What sort of product are you talking about?
The term MAIN is generally reserved for an overall Master volume control, and Output level control might be for several individual signals that a device is able to control. That might be for a mixer, or, I'm guessing on this particular forum, 2x Aux Sends from a mixer into your In-Ear-Monitors. The Output level controls how much of each signal you receive, and the Main controls the overall volume.