A great way to mask perfect shapes or blocks around your drawings is to use the Shape Guides. When a guide is active with the eraser, you’ll see a pink boundary line defining the edges of the guide - this is the boundary your eraser will “draw” in.
If you are using a pressure sensitive stylus such as the Apple Pencil, the mask will respond to the pressure of your stroke. This sensitivity means the stroke may not completely meet up with the boundaries shown - they display the maximum possible value. We recommend you use your finger to erase along the guide for a solid edge, or temporarily disable Pressure settings under Settings > Stylus.
The Slice tool does not work with the Shape Guides at this time. If this is important to you, please let us know.