If you specify the EXCESS=SUPPLY option, then there are three possible scenarios to deal with:
Drain the excess supply from all supply nodes.
Drain the excess supply from nodes that have missing D demand values. If a node has a missing D demand value, then the amount
it demands is determined by optimization. For a demand node with negative supdem
value, that value negated is equal to the sum of flows on all actual arcs directed toward that node.
Drain the excess supply from all demand nodes. If a node has a negative supdem
value, that value negated is the lower bound on the sum of flows on all actual arcs directed toward that node. If a node
has a missing D demand value, then the amount it demands is determined by optimization.