Tags are nested by merely dragging and dropping in the tag tree. Drop a tag onto another tag, and the tag (and all of its children) will become nested under the second tag. Drop a tag on the "Tags" label (it's not a real tag) and the tag and its children will become top-level tags. Stacks are merely collections of notebooks; they're nice because they allow you to organize your notebook list (you have a max of 250 notebooks -- managing them as a flat list can be onerous) and certain operations, like search can be applied on a stack of notebooks. Notebooks are merely flat collections of notes.Notebooks and stacks appear in the Notebooks list. Stacks cannot be nested: if you drag a stack onto another stack, the notebooks will be moved from the first stack into the second stack, and the first stack will be deleted. Notebooks cannot be nested in other notebooks. However, notebooks and stacks coexist in the root of the Notebooks list; stacks are there to allow you to organize that list better. The Knowledge Base is a work in progress, and more topics are being added.