It's generally for any part that's critical to the functionality of the blaster, especially those that have no easy, viable substitutions. (For example, rubber bands break more easily than good springs, so said springs would be much more beneficial.) Screws are lower on the list, but worth including. All parts should be parts that you can buy somewhere, not stuff like reservoir dimensions which are useless unless being used for 3D fabrication.
That said, I suppose I will leave it to those posting to decide what they consider is important.