Atomic Note
Each unit captures exactly one idea — one concept, entity, fact or process.
An atomic note is a note which contains only one idea and is self contained. The note then cross references to other atomic notes which are constructed in a similar way.
The working heuristics: split a note when its summary needs an "and", when only half of it would be linked from elsewhere, or when it mixes kinds (an idea plus its lookup table is a concept plus a fact). Merge notes when neither can be understood — or would ever be linked — without the other. Atomicity is what makes related links precise and keeps every view's pages focused.