@freedfromortalcoil If you add a clipping mask to a layer it makes it so that it'll only show up if there's something there on the layer beneath it
[ I didn't think my explanation was that clear so I added a lil demonstration too <3 - The layers are depicted in the bottom right & the little arrow thingy shows which layers have clipping masks/what they're clipped to ]
I've found it pretty helpful when it comes to trying to add shading, so I only have to pay attention to staying in the lines once xD