User Manual: Collision Categories
Because the editor is for making 2d levels, all shapes created in the editor has no depth, no thickness. In other words, the z coordinates of all entities are equal. Sometimes two visually overlapped shape entities may have different depth along the axis perpendicular to the screen. They should not collide with each other. To solve, the Collision Category is introduced.
Collision Category is often called ccat in short.
You can create and edit more ccats in the Collision Category View.
Shape entities has a ccat property. You can assign the ccat propery of a shape in the shape's properties setting dialog.
A ccat has one property: collide internally. If it is false, then all shapes in this collision category will not collide with each other.
You can set 2 collision categories as friends.
There is a hidden ccat. Its collide internally property is always true and can't be changed. And the hidden ccat can be made friends with other user created ccats in phyard builder.
Besides using ccats, there are some other methods to avoid collisions between shapes.