Nomen est omen in this case: Actions are exactly that - they initiate a procedure and/or control the way something is done; in most cases, this is linked to a change of the mouse cursor, the most prominent example of this being the SuperWeapons. Actions also decide whether a target is valid or not. As you can see from the list below, everything that generates a special cursor (and more) is linked to an Action, be it ParaDrops, the selling of units or simple guarding. The important factor here is that Actions can only ever be used once. While Types can be used again and again, a re-used Action will not work.
If the Action on a SuperWeapon is missing or invalid, the SW will be fired instantly on a random point on the map.
Type and Action
The relationship between Action and Type can roughly be translated to the one between Warhead and Projectile. Like the Warhead on a normal weapon, the Type of a SuperWeapon decides what kind of damage is applied, while, like the Projectile, the Action decides how that damage is delivered. (Be aware that this is only a simile - the "real" projectile, like the nuclear missile, for example, is part of the Type "Nuke" - it is independent from the Action "MultiMissile". Simplified, you can imagine the Action as being the part that controls which cursor to use.)
List of Actions
The following is a table of all Actions.
The Action "SellUnit" is a special case, as it was used before the YR 1.001-patch, but unused afterwards. It is mostly used either for SellUnit-SuperWeapons, which use SW-code to re-enable the selling of units. Note that if the SellUnit Action targets a unit in a Tank Bunker an Internal Error will occur when the Tank Bunker is destroyed or sold. As such you should avoid using the SellUnit Action.
Another special case is "AttackMoveNav", as it is not really used in the game; it can be accessed through the command bar, but is usually not used by anyone and can equally easy be removed from it (via ui(md).ini). If that is done, it should be safe to use it for SuperWeapons.
In addition, the Tunnel-Actions might be usable - RA2 and YR natively do not feature maps with tunnels, which makes these Actions technically unused. They are, however, part of the fully working tunnel-system which is actively used by the Terrain Expansion. Therefore, caution is required when considering to use them.