rationalskepticism.org

The reason it says "document number" and not "passport number" is because your passport is not the only valid form of ID needed to travel in the EU any more. You may also have a government issued national identity card and if using that it is that number you put in. At least that's how it is on the Ryanair online check in. There's a drop down menu where you pick which form of ID you're using and underneath it says "document number".

EDIT: I should mention that after I put in "Ireland" as my nationality the other ID option disappears leaving me with just the passport option so maybe UK does not have that option also?