I found a few posts:
Leo:
Sub-collections are collections inside other collections.
Like a sub-folder is a folder inside another folder.
Sub-Collections are collections that are below other collections. They aren't used that often, but you can use them to build a hierarchy of collections if you need one for some reason. Collection Members are items (files or folders) that have been added to the list of things that are in the collect…
So, to help me make it clear... If you right-click on a folder and 'Send To' a Collection as a Sub-Collection, then that folder becomes a 'place holder' with the folder name, and it's not the real folder. And if you delete it, you are not deleting the 'real' folder, of the same name/location? (but if you had made it just a Member, and tried to delete it, it would actually be the real folder, and you would actually be deleting the real folder??
And they are just a way to organize collections in more detail?
"use them to build a hierarchy of collections if you need one for some reason"
An example of why you might use them would be helpful. Like maybe you want to organize your pictures by 'date' in a sub-collection, and also the same pictures by 'location' in another collection, etc..? I seems that a sub-collection is the same as a collection at the root? Maybe the prefix sub- is confusing me. (like a folder is just a folder, even if it's a sub-folder)
Sorry for being long winded here..