Ammo is a mechanical and diagetic term. It is the total number of times an Item can be used before needing to be replenished, be it through bullets, arrows or energy cells.
Once an Item's Ammo has been reduced to 0, it can no longer be used for Actions that require spending Ammo until it is Reloaded.
If an Item is listed with no Ammo value, it does not use any form of expendable ammunition, and therefore these rules do not apply.