Sometimes the DC (Difficulty Class, see below) for an area does not fit a given roll. At the GM's discretion, a roll can be made "Hard" by adding 3 to the DC or "Easy" by subtracting 3 from the DC. This mixed with ADV and DISADV gives a lot of flexibility to tailor a roll to a situation without the mess of creating a new DC out of thin air for each circumstance.