The SUPPLY statement identifies the SAS variable in the ARCDATA=
data set that contains the supply at the node named in that observation’s TAILNODE
list variable. If a tail node does not supply flow, use zero or a missing value for the observation’s SUPPLY list variable
value. If a tail node has supply capability, a missing value indicates that the supply quantity is given in another observation.
It is not necessary to have a SUPPLY statement if the name of this SAS variable is _SUPPLY_.