Attack 1 and 2 refer to the two attacks that a unit has.
The Siege Engine is a good example in its upgraded form. In the Object Editor, in the Units tab: Human > Special > Siege Engine (Barrage).
See how it has two attacks.
Attack 1 can target only structures and debris. Attack 2 can attack only air units.
Whenever the Siege Engine attacks an air unit it uses the Attack 2 animation, using its barrage cannons in the back.
When attacking structures or debris (Attack 1) it uses the main forward cannon.