Using masks with 3D layers

You can draw a mask on any 3D layer in After Effects. The mask coordinates correspond to the layer coordinates in 2D space. However, the mask itself has no 3D properties except those of the layer itself, so you cannot draw a mask along the z axis. However, once you've drawn the mask on the layer, the entire layer can be animated along the z axis.

Note: Applying masks or effects to a collapsed 3D composition may make it behave like a 2D layer. To avoid this, turn off the Collapse switch in the Timeline window.