Boy+ is when you identify completely as a boy, plus additional genders. You don't feel partially boy, you aren't fluid, you are entirely a boy... plus more! Maybe you don't know specifically what that added something is, so boy+ is the best way to describe it. Or, maybe you have a lot of gender, but it all feels like it's simply additional while your main gender is Boy(/man/guy/male/etc). Just, Boy+. Boy, Bath, and Beyond. Y'know?