well correct me if im wrong but u can place units for different players...
not only player 1-12
so if u want to affect a specific player then under PU u have to read out for which player u have to place your desired unit/building to increase your supply...
PU = This player owns the building/unit
PA = This one get affected by the suplly change..
ADDITION:
and i allready posted a link to that list...
http://www.staredit.net/index.php?tutorial=105