Reference
reference material, reference filing, non-actionableDefinition
In GTD, information that is not actionable but worth keeping — a manual, a receipt, an article, a decision — filed so it can be found when a future action needs it, and never on an action list
The answer to is it actionable? — no, but keep it. Reference is the biggest bucket and the easiest to get wrong in both directions: actions hidden in it, and lists cluttered with it. PARA is a filing scheme for it; this site’s reference collection is its reference material, which is why that collection is private in full and why a public reference page is a curated exception.
Keep it because you will need it, not because you might. The test for a reference file is whether you have looked something up in it; the test for an action list is that nothing in it is reference.