DPM particles have mass, just not volume. The diameter is used to calculate drag and in the collision checks.
Euler is a scalar based model which then includes terms to account for granular viscosity & temperature (friction & bounce) as well as a packing limit.
The button you're looking for is "Help" on the multiphase panel, or the "?" top right of the Fluent main window.
You are navigating away from the AIS Discovery experience