Unit Attack Priorities

What determines what a unit given an "attack move-to" command will attack when it encounters many different types of units?

(I am trying to get units controlled by the CPU to attack different types of units in a different order. I.E. attack other ground units, then hero, then structures)

Thanks

the way that i would endup doing that is:

E: a generic unit is issued an order tagerting a unit
C: unit is owned by *CPU Name*
A: if number of units in ground units != 0 then
attack random unit in ground units
elseif number of units in hero type units != 0 then
attack random unit in hero type units
elseif number of units in building type units != 0 then
attack random unit in building type units
else
order (ordered unit) to attack move to point of last order
endif

of corse this is pseudocode.

that won't work, b/c orders to attack a unit make the attacker act differently than attack-moving does -- he will chase the unit forever.