Indeed... it's a category that IMHO lacks recognition. So hereby, on Akiba-Online, I propose to recognize these JAV performers as a distinct group, and further I propose to label them Pro-Am (professional amateur) or Semi-Pro.
The precise categorization is:
1. Pro: a performer who have made at least 8 (?) solo-vids "单体作品" (solo refers to single female performer, it doesn't refer to masturbation) [it's even tempting to lower the threshold to 6, I seem to find that once a girl has made 6 JAV's, she's very likely to go on to make tens of JAV)
2. Amateur: a performer who (usually) doesn't headline a solo-vid, she may only ever appear in a single JAV, or she might have appeared in over a hundred. Either way if she doesn't headline a JAV, she's amateur.
3. Semi-Pro: a performer who made between 1 and 7 solo-JAVs and then disappear from the JAV industry.
Of course as for any categorization system, there are exceptions, cross-overs and gray areas. But I think these 3 category does serve the JAV scene well.
There are a small number (tens?) of pro stars who make a significant portion of JAV, these have typically a "classy" looking cover with the performer's photo taking up most of the cover and her (pseudo)-name in large font. Many of these pros, during at least some portion of their career, release films on a once-a-month pace. That's not counting compilation vids which may further inflate her output rate. One prominent example is Julia, Blessed Be Her Bouncy Bosom, who have been release all-new solo-JAVs at a precise biweekly schedule for well over a year.
Then there are a large number of amateur (at least hundreds?) who make another significant portion of JAV (perhaps more releases per month than the pros). Those covers tends to be less "classy" and composed of lots of images (frames).
Finally each month if you look at the new solo-JAV "单体作品" (or check the covers with a large "classy" single performer photo and large font for her name), quite a number of them are of "new" or "newish" girls and if you follow these girls career, some of them will make a handful of JAV's during a "lazy" schedule of 12-24 months and then quietly disappear from the AV industry.
Compared to the amateurs these girls are hot enough (in face and/or body) or maybe some kind of minor celebrity status (e.g. athlete or musician) so they can command a whole AV vid, but end up deciding not to dive into a full career. From what we can gather these girls don't deviate too far from their "normal" career/life path, only made some easy money doing porn and seemingly "melt" back into mainstream society.
It would be an interesting exercise: go back to say 2012, take all the girls who have made at least one "单体作品" from that year and see how many of those girls are still releasing "单体作品" in 2015, I'd bet 50 cents the ratio is well under 50%. And most of these have not made 8 JAV's in their whole career before disappearing.