It's an extra or in this case a "short".
RadioKON no it was definitely a chapter if it was a extra or anything it would of said so
It does say so. It says Chapter 16.5 Extra. If it was a new chapter, it would be Chapter 17.
It's an extra because it really doesn't have to do anything with the story line. I've read quite a lot of manga to know what I am talking about. Mangakas normally put out 4-8 chapters (the entire volume) with an extra at the end. Those chapters usually revolve around an incident or trial. So, yes, this is an extra. The next chapter will start a new volume.
after a long wait :D <3 way to short !!!