In a loop system the supply line is the one that sends the hot water from the boiler to the radiation. Which was which was important when using standing cast-iron radiation because you would want the water to come in the bottom and go out the other side or the top pipe. With cast-iron or copper-tube baseboard the water can come and go from either side.
It is an long-told technical practice to put the circulator on the supply side and have the expansion tank between the circulator and the boiler. This makes sure that the circulator creates a positive pressure in the radiation line and does not create a low pressure on the suction side of the circulator.
This was not done for many years before on residential units because the circulator gaskets could not take high temperature water. It was always done on powerful commercial circulators because the circulators would destroy themselves from water turning to steam on the circulator inlet, causing cavitation damage. The commercial circulators were allowed to have dripping seals while being watched by maintenance crews.