CAT provides two basic modes of operation: Setup and Animation:
Setup Mode: In Setup Mode you create and modify your CAT rig. You can also add or remove bones later, even after animating the character.
If your character is already skinned and you want to adjust bones, first go to frame 0 and on the Skin modifier
Advanced Parameters rollout, turn off Always Deform. That way you can edit bones without affecting the mesh.
Animation Mode: In Animation Mode you animate your character.
Manipulation Causes Stretching. So, for example, if you enable this for the spine bones, then moving the ribcage mode will
cause the top spine bone to stretch.
Absolute layer in which to animate.
(Setup Mode).
The button image switches to Animation mode.

When you first create a CATRig, it’s in Setup mode, so you can copy and paste parts, apply manipulation gizmos, and so on. After you access Animation mode, you can switch back at any time by clicking the same button.