The erase area, like block and clear, is measured in a certain amount of 4x4 pixel squares arranged in a circle. The amount is determined by the pen size. The squares are aligned to the grid and cannot erase outside it, which avoids the possibility of pen-s 0 block.
The most common and obvious use for erase is removing objects from the screen. But it can also be used for making designs in block, such as thick straight lines, big capital letters and so on. Players can go even further by filling the voids with elements, and then erasing the surrounding block as a technique to make a large pixelated design out of elements.