A fleeting note -- inbox. Ideas sketched on a back of a napkin, in the note-taking app on the phone.
Why it is important to keep notes separated? It creates a certain ever-ascending cycle of development. Idea rises in a spiral trajectory and grows.
UPD: 220821 Additional thought, that support my opinion, stated by Alexey in his garden though his idea is related to GTD, but the concept remains no matter what.