Permanent Notes for me are smallest, indivisible unit of an idea / thought / procedure / project / category, etc that is made from Fleeting Notes taken during a call / meeting / reading / watching, etc. Typically these will contain a single idea and an exploration of it, which might be concise enough for a glance for smaller subtopics or large enough to require their own coffee for more complicated topics.
A map of content for an idea can be found in Reference Notes.