The block load is the simultaneous load on a set of rooms, zones or systems at a specific time.
Example:
Room A, Room B, and Room C all have 5 ton maximum capacities occurring in June, July and August respectively. On July 1st, the maximum simultaneous peak occurs at 2pm - with 3 tons, 5 tons, and 4 tons for Rooms A, B, and C. The Block load is then (3+5+4) = 12 tons