The topics are categories; they are used as tags.
Any note can be tagged with one or more topics.
The numbers don’t matter; what matters is that they are numerically sorted and numerically related.
The system structure and theory is roughly based on the ideas of Johnny.Decimal.
I initially took inspiration from Dewey Decimal Classification, but diverged when I saw its racist/sexist grouping and when I started embracing my own focus.
Categories are maintained here.