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.