A little more explanation, from the CTF codes guide:

F/A ("full ammo") - less important
When to say F/A: When you're "R" or "RTS", with some or all power ups available.
What to do when teammate says F/A: Nothing really, it just gives you a safer feeling.
Explanation: It's good to know if your teammate can defend oneself; the more ammo, the higher the chances of survival (so when he is R3 FA and you die, it's no problem).

H/A ("half ammo") - less important
When to say H/A: When you're "R" or "RTS", but with only some non-powered up ammo.
What to do when teammate says H/A: Nothing really, just be careful (don't die unless .
Explanation: It's good to know if your teammate can defend oneself; the more ammo, the higher the chances of survival (it's risky if you die while you're in a better state; more health and more ammo, than your teammate who is R).

N/A ("no ammo") - less important
When to say N/A: When you're "R" or "RTS", but with no ammo (or crappy ammo, like freezer and TNT).
What to do when teammate says N/A: Nothing really, just be extra careful.
Explanation: It's good to know if your teammate can defend oneself; the more ammo, the higher the chances of survival (it's risky if you die while you're in a better state; more health and more ammo, than your teammate who is R).