From Liquipedia Warcraft Wiki
The Attack Index refers to the two attacks a unit can have. A unit can have Attack Index one and Attack Index two.
If Attack Index one can target the target, it will always be used over Attack Index two. Otherwise Attack Index two will be used if it can target the target.
If only one Attack Index is enabled, it will be displayed on the left side. If two Attack Indices are enabled, Attack Index one will be displayed on the left side. For the right side to display Attack Index 2, it must have either different damage values or a different attack type than Attack Index 1.
Attack Speed and Attack Range can be different from Attack Index one.
Hidden Attack Indices
As explained above Attack Index two does not need to be displayed, if its damage values and its attack type match Attack Index one. The following units have a hidden Attack Index two:
- Alchemist: Can attack air units with an attack range of 500. Attack speed is same as his ground attack.
- Gryphon Rider: Attack Index two attacks air units, but has no splash damage.
- Heroes have a hidden Attack Index two, that allows attacking air at an attack range of 500. Other stats are similar to Attack Index one. This Attack Index is initially disabled, but can be enabled with Orbs.