Collections is a MODX Revolution Extra that adds a custom CollectionContainer
resource class with the following behaviour:
Just like the MODX Resource Tree itself, Collections supports nesting. You can create a Collection within another Collection. Sub Collection Containers will be displayed in the resource tree and their children will be displayed in the grid view.
You can drag n drop Resources into a Collections container and if they don't have children of their own they will be listed in the grid. If they do have children, they'll just remain in the Tree as usual.
As of version 2.x, Collections supports customizable views. Views are configured in Collections Custom Manager Page (CMP):
There are specific settings for Collections Resources vs Selections.
The official documentation for Collections can be found here: https://docs.modx.com/extras/revo/collections