A 3D layer lets you move in real depth instead of flat 2D space — it opens up perspective, rotations, and true dimensional movement.
Steps to use the 3D layer switch
In the timeline, click the Cube icon next to a layer to enable it.

New properties appear instantly: X, Y, and Z Position plus Orientation.

Move the layer in space using the Selection or Rotation tools — it now has actual depth.

One more thing
Pair 3D layers with a Camera for true perspective and realistic camera movement through your scene.
Related how-tos
How to use parenting to link layers together in After Effects
How to add and edit cameras in After Effects
How to use adjustment layers in After Effects
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