A couple of years ago I wrote The Big Ol' Genre Glossary. This blog post was intended to be "the last word" on the subject of genre identification, etc. Ha ha, is all I have to say to that.
As you might know if you ever go on internet writing fora, there are some genres and sub-genres that are just fuzzy, and there is a lot of overlap. Five different people might have five very different ideas about what constitutes "paranormal" versus "supernatural" versus "urban fantasy" versus WHATEVER. People can get very anxious about how to categorize their own work. I've had writers say to me, in tones of deadly earnestness, that they know agents will look for any reason to reject, and if they get the sub-genre of their own work "wrong," they will never be taken seriously and agents will hate them. That getting this bit of information "wrong" will be cause for an auto-reject.
Yes! Those terrible, dragon-like AGENTS. Always keen for any small reason to run writers through with a pike and roast them alive, the better for the feasting! Oh you didn't know we get younger every time a writer screams? Yes! Your agony is our elixir! Our blood is thrumming with your pain!
Oh, the guild tells me I was not supposed to say that last part aloud. Please strike it from the record. Also disregard the cackling. Now. In all seriousness.
For the most part... Agents are just people. People who love books, and who want to help facilitate the making of books. People whose job it is to advocate for authors. (That FOR is quite important!). We work with a lot of authors. We LOVE authors. We recognize that authors can sometimes be neurotic. We are not trying to drive authors crazy (or crazier, anyway).
This drama that the internet has cooked up about agents declining you because there was a typo in your query, or because you formatted an email query letter as a business letter complete with home address (or failed to do so), or double-spaced when you meant to single-space... or because you said "dark fantasy" when you meant "urban fantasy" or "paranormal" when you meant "supernatural"? Is just not true.
* First of all, as I mentioned, there is a lot of overlap, and different people have different definitions. If an agent was to decline your work based on that alone? They are not somebody you'd want to work with.
* Tying yourself in knots because of this kind of minutia may be keeping you from looking at the bigger picture, and at things that actually WILL cause agents to accept or decline your work. Things like having a killer pitch and tight, great, polished writing in the actual ms. This is truly what the agent cares about most.
* Some of the most interesting books defy easy categorization. If you have a book that is gorgeously written but also highly commercial, that is a GOOD THING. If as a genre it is something that lies on the crossroads between mystery, romance and fantasy (or whatever) -- that too is probably a GOOD thing, not a bad thing.
* If you have a magical story and you just call it "fantasy" I won't even blink an eye. The default "big category" of Fiction, SF/F, Mystery or YA is actually sufficient information. If you want to get more specific, that's fine. But if you go nuts with it and decide to make up a genre like "high-urban-splatter-steam-rotica"...well if it seems like you're joking, I'll chuckle. If it seems like you are being serious, I'll roll my eyes. But if the book sounds interesting, even that bit of silliness wouldn't stop me from continuing.
* IN fact I'll go further and contend that "overcategorizing" is self-limiting. If you've written what you consider to be "high-urban-splatter-steam-rotica"... you'll never find an agent who reps that, no matter how much research you do. If you consider your work a "Steampunk inspired paranormal-mystery" you might find a couple of agents who rep all of those categories. But if you just call it FANTASY, wow, suddenly you have a ton of potential people to query, and you can pick and choose who seems like might be the best fit from this larger pool.
* So sure, absolutely, give categorizing your best shot. But if you find yourself freaking out over which sub- or sub-sub-genre your work falls into, understand that this is of little concern to agents at the query stage. They care that the story sounds cool and the writing is excellent. And that is where you should be putting your energy.