The main idea of an outliner is the tree of notes, where each note can have subnotes. In comparison, in popular file systems only folders can contain subfolders and files.
You can, for example, move one note under another note, or one image under another image. This approach enables easy information management, because you don't need to think which kind of record you would like to create: note or folder?
Each notebook has its own tree of notes starting from the root (invisible) note. The trees can have unlimited levels and unlimited notes on each of the levels. I created a notebook with 1,000,000 notes for testing purposes and it worked as fast as small notebooks.
You can do anything you like with the notes in a tree: create, rename, edit, copy and move them. You can also drag note from one tree to another. Please note that when you drag notes from one notebook to another, it will be first copied to the new one and then deleted from the initial notebook.
When you drag, copy or move notes, they come with all their subnotes automatically.