See? I don't even have to give you the answers. We're trained to recognize the sub-genres literally by eye.
Okay, what's the plot/style/genre of this book:
You can click on any of these covers to see what the actual book is, but I'm guessing you will only need to click on that last one to see what the book is about. The rest of them, you know already.
Romance novels are designed to look immediately familiar and obvious. No subtlety, no cross-marketing, no ambiguity as to the genre, sub-genre, publisher (in the case of Harlequin Enterprises lines), even plot line.
Next time you're in the supermarket, try this little experiment. Go to the paperback book aisle, then go to the snacks aisle.
How much reading of the labels would you have to do to pick out the style of snack you wanted? Not much.
Same thing in the book aisle. And yes, this holds true to some extent with other popular fiction genres, but there the author's name can be very important. You might know it's a thriller but you might not buy it unless you see Harlan Coben's name on it.
Did you need to know the authors of any of the romance covers above? We're not being trained to look for any authors past a very small handful of famous names. And even with the famous authors you get both cues: Eloisa James's name is in large type, but the cover art still features the now-iconic backless ballgown suggesting a historical romance with some sex.
Finally, if you want to know why I picked that specific unidentifiable book, it's because it's got a claim to be considered a romance (HEAs!) and I wrote about it here. But it sure isn't being marketed as a romance, is it?
As an added bonus: our old favorite, the YouTube clip showing how tight the constraints are on cover design for urban fantasy/paranormal romances: