Drain is in the lowest point of tank. Draw off sump is situated at a higher point so that no water is allowed to escape with the fluid that is drawn off.
To elaborate on lynlynch's answer, the suction side of the drain pump is usually located in a sump pit 18" to 24" lower than the tank floor. The draw off is usually 12" to 18" above the tank floor.
These measurements are based on a 1 M Barrel tank and 12" suction side piping.
__________________
The first 5 days after a weekend are always the hardest................................