Collections represent resources which are contained within other resources.
For example, Nodes within a Node Group:
https://localhost/API/:version/master/NodeManager/objects/NodeGroups/:id/collections/Nodes
Or Files and Directories within a FileSystemItem:
Some collections allow the user to add or remove items from within it. Others are fixed and cannot be changed. Depending on the resource the collection may be managed by adding and removing a list of resource IDs or by adding and removing complete objects.