What exactly does "Skills fire an additional Projectile" mean?

I'm specifically wondering about the exact game definition of "fire" here. Terminology in POE tends to have very exact definitions, and this is a term I'm not familiar with that could potentially mean many things.

Does it mean whenever any skill creates a projectile, which would include skills like Ball Lightning and Lightning Trap? Does it only include the moment when a projectile is fired from a weapon, which would not include Ball Lightning, Tornado Shot pops, and so on? There are many more possibilities that I won't bother to list.

I'm hoping Endless Munitions in the new Ranger Deadeye class works with Ball Lightning for a massive AOE buff, or for any other skill that would play very differently with an extra projectile. I'm fine if it only makes attack skills like Lightning Arrow that much better, though. Right now I'd just like to know what to spend time theorycrafting.

Yes, 'fires an extra projectile' works with any skill that creates a projectile, 'fires an extra arrow' works only with bow skills (that create a projectile).