People called it “knotting au” or something to that effect, and/or listed/tagged it with a series of tropes that made explicit (heh, explicit) what they meant. There wasn’t a fixed set of tropes. Because it spread via anon fic memes, the tropes featured in each fic depended on what the prompter suggested and on what the writer made of those requests. So some prompter could ask for “knotting au, heat, bonding, impregnation,” and different writers could take all of those tropes into account, or pick and choose which ones they wanted to tackle and/or add different ones. Other prompters would call for a different, more or less overlapping, set of tropes and so on. And the same prompt (the setting/premise) could be spun into very diverse directions. There was a fluidity to it that became a hallmark of the genre, so that each iteration could bring forth a new variation.
Netweight (2013). The Nonnies Made Them Do It!
Have you ever looked at the Alpha/Beta/Omega Dynamics tag on AO3 and wondered how on earth we got here? Netweight has the answer, and the answer is Supernatural anon kinkmemes! In this meticulously researched piece of fannish meta, netweight traces the history of what eventually came to be known as A/B/O from its murky origins in some of Supernatural fandom’s anonymous online spaces, to being named, to hopping the fandom boundary and becoming the cross-fandom shared universe phenomenon it is today. She provides plenty of context for those of us who aren’t in SPN fandom, lots of links to primary sources, a bunch of useful timelines, as well as some good caveats on the limitations of the research. “The nonnies made them do it!” is a great effort to document and preserve a piece of fannish history which has puzzled many fans and fan studies scholars alike.