Connected to attacking (X*256, Y*65536). 
"When this is set to a certain point, a unit will face in the direction of that point while it idles. This has no effect on the unit when it's moving. By default, it was on the current position of the Marine. When the unit is attacking another unit, this point is set to the unit's target. This is likely used to make a unit face its target when attacking." -Roy