18 yo daughter has just requested "AFTER", a fanfic based upon 'the boys' in one Direction, that got published.  In reading the reviews on Amazon since there aren't any here yet (the series of 3 books has just 2-3 wishers per book on PBS WL), they are calling this series "NA" for college-age young adults.

Can someone explain if there are true differences? IS  this a new sub-category?

There are some angry reviewers on Amazon due to the abusive dating relationship in AFTER as this book certainly does NOT sound appropriate for younger teens.

Yes. NA, New Adult, is a new sub genre. It is written with the mood of YA, but with more sex.

This is getting very messy for a book seller point of view. How does these get shelved? There are the younger YA, the older YA, and now the NA.

Wikipedia:

New Adult (NA) fiction is a developing genre of fiction with protagonists in the 18-25 age bracket. The term was first coined by St. Martin's Press in 2009 when they held a special call for "...fiction similar to YA that can be published and marketed as adult—a sort of an 'older YA' or 'new adult'."^[1] New Adult fiction tends to focus on issues such as leaving home, developing sexuality, and negotiating education and career choices.^[2] The genre has gained popularity rapidly over the last few years, particularly through books by self-published bestselling authors like Jamie McGuire, Colleen Hoover, and Cora Carmack.^[3][4]

The genre was originally met with some criticism, as some viewed it as a marketing scheme,^[5] while others claimed the readership was not there to publish the material.^[6] In contrast, others claimed that the term was necessary, with a publicist for HarperCollins saying that it "is a convenient label because it allows parents and bookstores and interested readers to know what is inside".^[7]

Examples of books in the new-adult genre include Jamie McGuire's Beautiful Disaster,^[8] Colleen Hoover's Slammed,^[9] and Cora Carmack’s Losing It.^[10]

NA Has been around for over a year now. But from what I gather NA is about college and sexual situations. I have no idea how one direction fits into that category. 

Jennifer, reading Jeanne's description above, I can see why it may be NA....  abusive dating relationships...  might be too much for true YA readers to handle.  I'm not familiar with the story, but just thinking...

The boys in One Direction are actually in their 20s, so I would just classify it was NA since that would be covering ages 18-25 or so.