Everyday full real life actions of experience provide us with a sense of the world and enable us know the immediate effects of our surroundings.
If we want comprehend the true nature of universal gravitation we need to find something in our full real life actions that is similar to energy exchanges of the whole universe. The interaction of our own bodies with the atmosphere gives us a sense of all energies acting together to produce an effect. By comparison we may comprehend the energies interchanges of the whole universe as having the effect through the motions of all masses of producing the gravitational interactions observed.
From this comprehension of the nature of gravity as energy interchanges of the whole universe we can begin to understand function of mass in these exchanges and to fully represent what is physically happening in Modified Newtonian dynamics with a varying mass function.