Here you'll find how to share elements (video layers, compositions etc.) in order to use the same content in multiple places, as it is done with pre-compositions in After Effects.
This feature allows you to do a lot of procedural effects and to rationalize your compositions. Here is an example in which the same composition is used as an image, a texture for a 3D model and a displacement mask :
Why sharing an element ?
Sharing an element, beyond being quite useful for compositing in general, is also quite useful when it comes to optimization and performances. As a matter of fact, sharing an element and re-use it allows the GPU to do less computation and less hard drive acces and use less VRAM (and it become crucial when we are talking about video layers).
So if you want to put the same video stockshot everywhere in different parts of your composition, you'll need to share it before putting it everywhere.
How to share a Layer (composition, video, image etc.)
To share an element : right click -> Share Element
This will result in turning the current layer into a reference (02) and creating its source (01)
Now you can use Ctrl+drag and drop to create a new reference
If you want to import one of the reference : right click -> Import
This will result in turning an instance into an independent layer :
If you shared only an image and you want to turn your source image or video into a composition, no problems, just select the source and turn it into a composition, either with right click -> Make Compo or Ctrl+Shift+C.
Now you can add any new media you want to to your shared composition :
How to use it
Well, identically as After Effects' precompositions, your source will stay the same but every one if it's references can be modified in any kind of way (masks, modifiers, placement, ect). Inside the example composition you can find "Procedural Terrain Generation" (also in the tutorial part of the documentation) in which two shared compositions "Color" and "Displace" are used in many places to set up the mountain texture and relief as well as the trees size and orientation.