As suggested by the acronym "DPM - Discrete Phase Model". Fluent injects a discrete mass flow rate.
m_dot = rho*vel*Area is valid for a continuous phase

However, for particles the mass flow rate is
m_dot = # of particles / sec * mass of particle

That is why you need to both specify mass flow rate and velocity in the DPM injection panel. The latter has nothing to do with the above mentioned formula, so should be an independent input