@koanhead, @quarkquarkquark Yes, mass and energy are two different aspects of the same thing. So, it is this difference that i am referring to as mass properties and energy properties of the same mass-energy.
So, you can always take some energy and say “I could convert it all to mass” or do the vice versa. However, there are perceivable differences in properties of mass-energy when it is in mass state and when it is in energy state. Hence,when equal amounts of matter and antimatter interact. all mass properties of this mass-energy get cancelled and only energy remains. This is what i mean by mass properties and energy properties of mass-energy. And I am not using any units, just appreciating the different aspects of the same thing.
Even though we know about the mass-energy equivalence, that should not stop us from appreciating the differences in something that we can clearly identify as having mass and something that we can clearly identify as having energy using our 5 senses. (i.e. think how you saw world when you were unaware of E=mc^2 and things like that. You could say then, that a rock is something that has mass and a bolt of lightning is electric energy) :)