The Polydispersed Fluid tab for a domain object contains settings that define the properties of polydispersed (MUSIG) fluids. It is accessible by selecting the Polydispersed Fluid option for Fluid and Particle Definitions... > <fluid definition> > Morphology on the Basic Settings tab.

For details, see Polydispersed, Multiple Size Group (MUSIG) Model in the CFX-Solver Modeling Guide.