Specifically the Ruleset Reference Nightly. Yeah, sure, most of the information is the same, but TFTD adds new stuff too. Or different numbers that don't appear in the reference. For instance, Mag Ion Armor has a movementType of 3, which isn't listed in the Reference, as OpenXCOM doesn't have to deal with moving underwater. There are just so many little things like this in the files that don't appear on the Reference page and it makes it difficult to create mods when you have to play trial and error with things you know nothing about.