"Make main category"

Hello,

I’ve noticed somethhing in my wordpress 4.2.2, it may be new, or not, I have no idea. Maybe I didn’t notice it before.

Next to my categories in my post editor, I have something saying “Make it the main category” next to categories.

See (in french) : http://www.noelshack.com/2016-13-1459542473-untitled-1.jpg

Is it something from the default wordpress install or a plugin is causing this ?

Thank you