I know what it is,
IF the player has a BA_BeamLaser then the player drops the laser in favor of the weapon being purchased.
That is all.
IE, lets say you bought a man pack PPC, but now want to purchase a Bear Autocannon or flamer, IF you have a BA_BeamLaser in your inventory, it will drop the beam laser, and give you the bear autocannon or flamer that you wanted to purchase.
At this point though, if you do NOT have a beam laser in your inventory, it will reject your ability to buy the weapon as your inventory is too full, but the code will always assume that you want to drop the beam laser if your inventory is full.
Though I am only assuming the "inventory full" code as I dont see it specifically there.