What are some of the notable points in the derivation of $M\vec{A} = \vec{F}_{ext}$?

(N/A) $(1)$ To determine the motion of the centre of mass,no knowledge of internal forces of the system of particles is required; only the external forces are needed.
$(2)$ To obtain this equation,we did not need to specify the nature of the system of particles. The system may be a collection of particles with various internal motions,or it may be a rigid body undergoing pure translational motion or a combination of translational and rotational motion.
$(3)$ Regardless of the nature of the system and the motion of its individual particles,the centre of mass always moves according to the equation $M\vec{A} = \vec{F}_{ext}$.