What makes [difference between upper and lower MG]

My two cents:

Lower MG is written for an audience of kids who are probably eight to ten, with main characters slightly older than that. The word counts tend to be shorter. The subject matter also tends to be lighter. You're not going to see anything even approaching graphic violence in a lower MG book. There probably will be little to nothing in the way of romance.

Upper MG, on the other hand, is longer. In fantasy, it's common to see these books run in the 70,000 to 80,000 word range. You are likely to see characters who are 12 to 14. I'd say Percy Jackson and Fablehaven start out as upper MG. Harry Potter starts out right on the line between lower MG and upper MG. The next few books are upper MG, before it turns into YA with the fifth book.

You can get away with a lot more violence in upper MG. You don't want to describe things too graphically, but characters do die. Even non-graphic torture (e.g. the Cruciatus curse in Harry Potter) can show up. There might be a little romance in upper MG, but you won't go much beyond holding hands and quick kisses.

'Game of Sunken Places' by Anderson M.T. I thought was pretty good and that's lower MG, I believe. The kids are 12 years old, if I remember correctly. No romance, but magical and "scary" creatures. Fun adventure. I enjoyed it.

Skary Childrin and the Carousel of Sorrow by Katy Towell is lower MG too I believe that has some weird "skary" stuff in it, but I did like it in a weird way.

The middle grade category is generally considered to be from ages 8-12. My understanding is that HP is middle grade through book three.

Twelve and up is YA. Those books that cross between these age ranges are labeled young YA or Upper MG. So if the book is for ages 10-14, it could be called either. The publishing house will often decide based on which category they believe will garner better sales. Bookstores may shelve it as they think best for sales too or as the political and social climate in their communities dictates.

The classifications are all about marketing. In work for children, there is also a fear that a child will pick up themes that are over his or her head. Reading level is also considered. That's why kids get a separate section in the stores.

11 - 13 fits into upper MG, but it's a rather narrow range. The publisher will want the market demographic to be as wide as possible. They won't want a book they can sell only to kids within a two-year age span. Besides, it's not like you can prohibit ten-year-olds or fourteen-year-olds from reading it.

I think you are better off not narrowing the age group. If you are sure it isn't YA, just call it middle grade. Let everyone else worry about upper or lower. Even if they decide it works as young YA, it doesn't matter. What matters is whether they like the book and think they can market it.

Caveat - it does help for the author to be in the ball park because that shows the author has some knowledge of the industry.