Complex sentence: The standard MPX submachinegun comes with a rear and front flip-up sight that is attached to the integrated rail system on the top of the weapon, an optional extra is a SIG reflex sight mounted to the rail instead of the flip-up sights.
Split into simpler sentences: The standard MPX submachine gun comes with a rear and front flip-up sight that is attached to the integrated rail system on the top of the weapon. One available modification is a SIG reflex sight mounted to the rail instead of the flip-up sights.