What is module selling and how does it work?

I don't need explanation of modulescripts. Someone talked about it once here and I don't understand do you sell the asset id? I think that would be pretty dumb because people could just leak the asset id. Do you sell the actual model? I'm really confused so can you guys explain it to me.

Selling the Asset Id? No

How do modules work? The owner of the game using the module ID must OWN the Module Model Script

Do you sell the actual model? Yes.