Subs don't have a bombardment range like destroyers, cruisers, or battleships. Their range is 5 km the same as any other units in melee close combat.
For subs:
Hold fire will allow you to sneak through an enemy blockade and spy on them without engaging any enemy units. Unless you are spotted by a naval bomber or spies you are invisible.
Return fire doesn't really work well for subs.
Fire at will means that your subs will follow your orders, so if you tell it to move it will move, if you tell it to attack it will attack. If it is sitting still it will attack units that stumble across it. So if your subs are moving while set to fire at will they will not engage the enemy.
Aggressive fire means that your submarine will ignore your orders and attack the first thing in it's path. So if you are moving back and forth in the strait it will stop moving and attack an enemy unit that gets within 5 km.
