What is a mortar box?
A mortar box is exactly what the name states. A box your going to fill with mortars.
Each box, consists of two long sides and two short sides, which form a frame with no top or bottom when assembled. These wooden frames are assembled using metal rods, washers, and cotter pins, then filled with sand.
These boxes are usually only used on the 4th when there are simply not enough racks to go around. Sand boxes take much longer to fill than it takes to cleat an equivalent number of racks.
Another reason mortar boxes would be required is if you are shooting out of steel tubes. By law, the steel tubes must be buried 2/3 of their length in the sand in case of a tube fragmentation.