What determines whether Destiny enemies are vulnerable or resistant to a particular damage type

EDIT: With the announcement of the Taken King DLC, a new subclass for each class has been revealed, coming this September:

• Titan: Sunbreaker (solar)
• Warlock: Stormcaller (arc)
• Hunter: Nightstalker (void)

Note: The following information was true upon original release of the game (pre June 2015), but new subclasses have since been announced.

This slot is due to the way subclasses were supposed to work. The mechanic I'll describe below was removed on the day one patch:

Removed subclass locking from builds.

Basically, the way subclasses were supposed to work was as follows:

1. You level up a subclass all the way, choosing which perks you wanted.

2. The final ability "locked" the subclass (can't change perks anymore) and gave you a slight boost to stats

3. Now that you've locked this subclass, you can start leveling a new subclass (either the same or your other option) with an exp bonus to level it faster

If that wasn't clear enough, see this article

The point of this was to experiment with different builds. They removed this feature in favor of freely swapping between subclasses. It seems it was an oversight to leave the third subclass slot in the character menu.

I'll convert my comment to an answer as it was getting long.

I get the feeling playing the game that precision kills drop more ammo, but have no evidence.

Do be aware some guns have the 'surplus' attribute which means more ammo will be found for that weapon slot.

Thanks to @Havatra for the thought. There might indeed be Grimoire cards for this. Here is an example of an area bonus earned through killing enemies in that area. So far I have seen bonuses to, EXP, Weapon EXP and Materials, but I don't have a huge Grimoire score and http://destiny-grimoire.info/ doesn't have the card bonuses listed.