These are expressions, you use them inside your event and/or actions..
so instead of:
system set ammo: ammo+30
system add to ammo: 30
system set ammo: min(ammo+30,200)
system set ammo: clamp(ammo+30, 0, 200)
you could also use your normal way and add an every tick event setting ammo to clamp(ammo, 0, 200), that way it can never get below 0 or above 200..