I have a link! It's--hard to explain, but teh best explanation I can give personally is that it's what appeals to your hindbrain in reading, like, IDK, hurt-comfort or your particular kinks, and choose to read/write/both fic based on those things.

Slash shock, shamelessness, and a rec. which has pretty good context and explanation of the idea of id-fic. Tell me if you need more; I forgot about this link and am enjoying a re-read myself!