Types of Add-ons

Hi All,

I'm a bit confused about the term "add-on." I was the under the impression that only if the functionality is inside B1, then it can be called "add-on." But it seems there are lots of add-ons that are developed and live outside B1. Can you please clarify how can one tell where does the add-on live? Is there a different name when it's all inside B1?

Thanks,

Alaa