3D Compositing with Action > Action: Sources >
About Source Nodes

Source nodes can be used to separate front and matte clips in media and then apply separate transformations to each clip. For example, if you apply media to a surface and you want to create an effect where the matte moves into the scene, add a source that isolates the matte clip. A matte source lets you animate the matte's position separately from the front. You can also use source nodes to apply several matte clips to a single front clip.

Use source nodes in Action to create more advanced techniques such as transforming or replacing a front or matte clip or applying motion blur.