In response to this tweet, a link from Sascha who "distinguishes [only] between content notes and ... notes on notes". But it appears a little more complex than that.
"A Zettelkasten is neither a neatly structured filing system ... nor a turmoil deep sea generating ideas out of the ununderstandable chaos. There are three layers in my archive..."
That top level can emerge organically/bottom-up, or be created deliberately.
It sounds complex (and the post needs diagrams), but as it was developed piece by piece, as required, it probably doesn't appear complex to the author. The final remarks are spot on:
"If you chose software that doesn’t deal with those layers in a sophisticated way, you will not reap the benefits in the long term.... [and miss] the opportunity to create something that is greater than you. "
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See also: Zettelkasten (Overview)