You can place composite shots inside other composite shots. This is called embedding. The embedded composite shot is often referred to as a 'child' while the composite shot containing it is known as the 'parent'.
Embedding opens up advanced compositing workflows and also provides a way to organize your layers into distinct groups. Using embedded composite shots can also provide a performance boost, when combined with the proxy system.
To embed a composite shot simply add it to an existing composite shot as a layer, in the same way you would add a video or any other asset.
Masks and effects can be added to embedded composite shots just like any other media layer.
Creating Embedded Composite Shots
When you convert a layer to a composite shot you are automatically embedding that new composite shot.
You can also select multiple layers when creating a new composite shot, which is a quick way to group related layers together inside their own composite shot.
Impact on the Render Pipeline
The contents of an embedded composite shot are rendered as one, so that the embedded composite shot exists as a single layer in the current timeline's render pipeline.
Therefore you can use an embedded comp to 'bake in' all of its contents. Subsequently you can then use the embedded composite shot as a source layer for effects, knowing that they will use all of the embedded composite shot's content.
If an embedded composite shot is set to the 3D unrolled dimensional state, any 3D content inside it will be rendered in the unified 3D space, while still only occupying a single layer on the timeline. See The Render Pipeline for details.
Embedded composite shots can be combined with the proxy system to provide a major performance boost when working on complex projects.
For example, if you have completed work on a complex particle simulation which doesn't render in real time, in many cases you will be able put it into an embedded comp which you then proxy. This will perform a background render which will generate a faster performing proxy file, making it possible to continue working efficiently in the parent composite shot.