This excerpt taken from the PCG 10-K filed Feb 18, 2005.
Uninstructed Supply Deviations
Uninstructed Supply Deviations are the difference between scheduled supply and metered supply plus an ISO allocation for transmission losses. If Utility's net supply deviations(2) are positive (meter exceeds schedule), to the extent not needed to compensate a negative uninstructed retail load deviation, it is considered that excess supply was a Utility market sale and will not be included in Utility Supply for pro rate calculation purposes. If Utility's net supply deviations are negative (schedule exceeds meter), to the extent not balanced by a positive uninstructed retail load deviation, it is considered that Utility had to procure additional supply from the ISO real time market. The negative supply deviation quantity procured from the ISO real time market is considered a Utility market purchase and the quantity will be included in Utility Supply for pro rata share calculation purposes.