A literary genre (Wikipedia) is a category of literary composition. Genres may be determined by literary technique, tone, content, or even (as in the case of fiction) length. Genre should not be confused with age category, by which literature may be classified as either adult, young-adult, or children's. They also must not be confused with format, such as graphic novel or picture book. The distinctions between genres and categories are flexible and loosely defined, often with subgroups.
I write young adult, but I don't write with a specific sub-genre in mind. Defining the sub-genre of my last manuscript got a little tricky, probably because it could fit into more than one sub-genre depending on the way its presented. The toughest choice was between Paranormal and Urban Fantasy. Sure, there's a distinct difference between the two - paranormal is a romance sub-genre with fantastical elements and urban fantasy is a fantasy sub-genre primarily set in a city. BUT, my novel is also young adult, which means there is a healthy bit of romance thrown in there too (I know, not all YA novels have romance, but I'm pretty sure the majority do).
Romance with fantasy or fantasy with romance. It would be easy to decide if the romance element was the strongest aspect of the MS, or if the romance took a backseat to the fantastical. Unfortunately (for my sub-genre assignment task), romance and fantasy are given equal weight. So, I checked the genre for YA novels which I felt might be in the same category as my MS. The first books I looked up were Evermore by Alison Noel and Fallen by Lauren Kate and guess what? They are both categorized under paranormal AND urban fantasy. Is that allowed?
In the end, I'm not sure how important it is to get the sub-genre exactly right, especially if the MS doesn't fit perfectly into a single sub-genre box, but I wanted to emphasize the fantasy elements while querying so I went with Urban Fantasy.
What about you? Have you had trouble defining the genre or sub-genre for any of your works? How did you decide?