I've just updated my site with a new section all about projects! The way that it works in my build is via metalsmith-collections and metalsmith-pager.
First to identify that a post is going to be about a project (rather than some other topic) so that it will appear in my special 'Projects' page, I have to assign the post to a specific collection in some way so I can reference it later.
To do that I've use the following metadata in the post.md file:
--- collection: projects ---