"reference"

I use references only on projects. They're bits of information that I want to associate with one or more projects (a reference, like any item that's not a Value, can have more than one parent). In my system, they're usually either hyperlinks to relevant web pages, or DOIs to journal papers, or a copy & paste from some news item.

It's a bit of information that doesn't require any action in itself, but which is useful to have around when I'm viewing a project.

caveat: It's a while since I read the GTD book, so the way I use them might not be the same way as in the book

Regards,
Andrew