Webmention is a web standard mechanism that allow bloggers to refer to each others posts, from one site to another. When you link to a website in a Item/Post, you can opt to send it a Webmention to notify it.
The protocol is implemented by many publishing platforms, b2evolution being one of them.
In b2evolution, Webmentions are controlled separately for:
For more information about Webmentions, you can head on over to https://indieweb.org/Webmention.