When you have selected units that have a weapon, you can Right-Click on any enemy unit to order an attack. The attack command can be given on the map and mini-map. Once the targetted unit is destroyed the command will be removed.
Select your units, then press "A" and click-and-drag a circle you want to attack. This works for all units
Queue up multiple attacks
While holding down SHIFT you can issue multiple attacks in a row. All units will attack the set target in that order. When not in firing range, units will attack all other units that come into range. The attack command will ignore firing stances "Return Fire" and "Hold Fire".