From Outscape Wiki

Ordnance is the term in game for Limbalt stored on a ship in special Ordnance Modules. Each weapon that requires ammunition will deduct a certain amount from the total Ordnance stored on that ship or fleet. This is referred to its cost in "Shells" to fire.

As of Alpha 3: Currently 1 Limbalt = 1 shell. When there is insufficient Ordnance left in the fleet to fire any given weapon on any given ship that weapon becomes inactive but is not destroyed.

Warning! it IS possible to design a ship without Ordnance Modules! But this is an embarrassing waste of time....