Not sure what's confusing about it, but then I've been playing around here for a while, so maybe I take it for granted. I'll try to explain.
I believe Dom and Sub currently are image types, but not used in the game. (Dom might be used in Torturer, I'd have to check, but I really don't think it currently is) If at some point image types are changed in certain activities so that all of these are active, I imagine it will be Dom is Dominant, Sub is Submissive, and BDSM is generic, like a picture of a girl with some BDSM gear that could go either way.
Deepthroat, Eatout, Dildo, Strapon, Les69ing, Lick, Suckballs, Cowgirl, RevCowgirl, and SexDoggy are exactly what they sound like. They are specifically supposed to be images of those types. But again, there aren't any jobs hardcoded into the game that use these at the moment. To get them to run, you would need a script. (Don't ask me mate, I'm a writer, I can barely change minor things in the code)
Something to keep in mind is that most of these image types were added recently with the idea of giving the images more control over specific actions. It'd be cool to have a deepthroat scene where you probably get a deepthroat picture. Most of these aren't active, I believe. That aside, each of them, and all the other tags, have fallback tags too. For example, if it reaches for Deepthroat and doesn't get it, it will go on to Oral, and then on to Titty, and then on to Nude, and then Profile. (As an example, since I can't recall the specific tags off hand) Basically, when these are coded in or you see them through a script, it will reach for the image, and if that girl doesn't have any images of that type, it goes through a list of possible secondaries, starting with those most closely related. That ties into the Dom/Sub/BDSM pictures too. Imagine, for instance, you had a girl in a job that displayed the Dom or Sub image, but that girl had no designated images for that, so instead it tried to pull BDSM, didn't have that, so it tried Sex/Anal/Oral/Nude/Profile etc. however it goes in the code.
Another thing to keep in mind is that this is largely moot. Because the tags are so new, and most of the girlpacks on the site have been established for a long while, the player content creators making them doing things a certain way for such a long time, you're not going to find many girlpacks pre-set with these image types anyway. There are a few, from folks that are consistently updating or creating packs. (Not me personally, but that's largely because while I find the concept cool, I'd like to find a way to simplify things.)