d is sometimes used to represent an infinitesimal change in a quantity and sometimes a small amount of a quantity. E.g dx vs dM. dV could mean a small volume element and also an infinitesimal change in volume. How can it be used for two different things? My suspicion is that while converting Riemann sums into an integral, quantities inside the sigma like the finite mass mi of something is converted to dm for the sake of convenience and less notational clutter.