So I was looking more into sub-jhanas, trying to get Daniel's notation straight. Then I thought, this really does help explain how one jhana can be very weak and subtle, to extremely powerful, and have all sorts of flavors. When people talk about getting into very hard jhanas, maybe they're getting higher up in the sub-jhana of the jhana. I've never really looked for this in meditation before, but maybe in sessions where the jhana seems strong, in particular, to the exclusions of the sensations of the "outside world", it's because I'm hitting a higher, maybe formless sub-jhana. In particular, I just had this experience during the A&P where the feeling of my body became almost undetectable, and the space around me felt tangible and immersing. So, perhaps under this notation, that could be an experience of 2.5 (or even possibly 2.3)?

Some more speculation: maybe it is control over the first sub-jhana aspect that allows one to deepen and continue to remain in stages, like purposefully delaying the jhanic arc from arising by "holding back the floodwaters", for example, in the 2nd jhana. I find that in the 2nd jhana (or the A&P), there is definitely an aspect of the waters breaking loose and carrying things onwards, and that remaining at this point requires a lot of control. Maybe this control is a factor of the 1st sub-jhana, that being effort.

I'm speculating a bit, but applying the jhana/sub-jhana idea to my previous experiences does seem to show some connection. Now there are a bunch of fun experiments for practice, theoretically!
-Staying in one jhana and trying to ride the sub-jhanic arc up and down, then go on to the next
-Switching the place of a sub-jhana and jhana, for example, getting into 1.4, then tuning into 4.1
-Calling up fruitions from 1.4, 2.4, 3.4, etc...

Who knows?