You will often hear the terms "block load" and "room by room" as relates to the calculation of design heat gains and heat losses of a building. The heat gains are the summer cooling loads from the roof, walls, windows, lights, people, and equipment (usually in Btu's per hour) which result in the "tons" of air conditioning required.

When you analyze a building as if it were one big shell with no room divisions, you are entering the data for that building as a single room, and that is called doing a block load. All software that can do room by room calculations can automatically do block load calculations, because a block load is just all building data entered as a single room.  Some software can ONLY analyze one room per calculation. That type of software is typically known as block load software.

Room by room and block load software will calculate the same total building load because all the quantities of roofs, walls, windows, and other items will be equal. The benefit of doing a room by room calculation is that the loads will be separated out for each room, and that will allow individual room air flows (typically cfm values) to be calculated.

You can size your cooling/heating equipment properly with just a block load, but if you want to be able to design a duct system for the building, you must have room by room loads calculated. Room by room load calculations require more detailed information (you have to know individual room dimensions) but the information gained from that is well worth it.