Cheezus wept. I'm so glad I am not in a position of having to put up with this utter wankery.
I am not familiar with it, and the young women I know are not familiar with it either. It seems very 'Love Island!' A man seeing a woman, but not being 'closed off,' so if someone else comes along, and he fancies her, he can ask her out, because he and his other squeeze are not 'exclusive.'
They are not exclusive until he puts on a big show of fucking rose petals scattered around the villa, and a big bottle of wine, and asks her to 'be his girlfriend...' 🙄
Fuck that shit.
As a pp said, even in the mid 2020s, this ludicrous shite is still around. Young women waiting to be asked by a man to be his 'girlfriend.' Basically waiting for a man to want her, to desire her, to love her.... FML! 
When DH asked me out and I said yes, (a hundred years ago, OK about 38 years ago!) and we arranged a second date, that was it, we were dating. If I had caught him piss-farting around with another woman, I would assume we were 'off' and would have waved him goodbye and moved on. He never did that though because, well... he was with me/dating me. I was his girlfriend...
It is clearly a thing now, (this 'we're only seeing each other, and not boyfriend and girlfriend' shite,) as some posters are saying they have recently known it happen. But as I say, this doesn't happen to the young women I know. None of them would put up with it truth be told.
As has been said, it seems to be largely in favour of the man to not 'commit' properly, until HE is ready... Funny that. 
As I said, fuck that shit. Young women need to stop allowing this to happen!