what is hometree? vs. flat?

I've gleaned some vague concepts and data models regarding hometree/flat and collection nodes.

I've read the XEP-0248. Ok, but flat/hometree are ejabberd specific and I have no idea. I see that database column in pubsub_node called "parent." What's that for when there's a pubsub_node_option for "collection?"

And what does the node's path matter? Do I have to create a node as 'collection/sub-collection/node'? Or can I just specify the node's collection and create 'node'?

Is there anywhere to read about these things? The best that I've found is a vague reference to hometree being an implementation of "simple tree." Oh. Ok. How does that relate to ejabberd?

Please help.

Stumbling in the dark.