SB193 is restricted to Military/Law enforcement sales only by FN.
You do not HAVE to use subsonic ammo with a supressor but then the supressor will only function to limit the amount of flash that you will get from the muzzle, and quiet the powder explosion. You will however hear the supersonic crack of the projectile as it surpasses the speed of sound.
You HAVE to use subsonic ammunition with a supressor if you want to have the firearm as quiet as possible. The supressor will limit the explosive sound of the powder and the subsonic projectile will not break the sound barrier therefore you will not hear the crack of the sound barrier being broken. Quiet as a field mouse (well maybe a very large field mouse).
If you use subsonic ammunition without a supressor, you will hear the sound of the powder explosion as the projectile exits the muzzle but you will not hear the supersonic crack of the projectile breaking the sound barrier.