An evergreen note is written to be edited. That single decision changes how you write it.
A post is addressed to a reader at a moment: it has an occasion, a hook, and a shelf life. A note is addressed to you in two years, and its job is to still be true then. So it drops the occasion, states the claim, and expects to be rewritten when the claim stops holding.
The practical difference is what you do when you learn something new. With posts you write another post. With notes you go back and change the note — which means the pile gets better over time instead of just longer.
This only works if notes are small enough to revise. See [[Notes should be atomic]].
[!tip] Rule of thumb If revising a note feels like rewriting an essay, it is too big.
Related: [[Backlinks change what you write]], [[Why gardens beat blogs]].