VeryPoor These three text values correspond to DesignBuilder's Cracks Templates, which provide typical crack flow coefficients and exponents for different envelope tightness classifications. If None, the exterior airflow leakage parameters will be derived from the room infiltration rate specified in the room's energy properties, which are in units of m3/s per m2 of facade. This derivation from the Room's infiltration will compute air leakage parameters for exterior cracks that produce a total air flow rate equivalent to the room's infiltration rate at an envelope pressure difference of 4 Pa. This default derivation is not as complete of a representation of building ariflow dynamics as the DesignBuilder Crack Templates are. However, since the airflow leakae parameters are derived from values in m3/s-m2 of infiltration, they are easier to relate to the results of infiltration blower-door tests, which typically express infiltration rates in these units.