Definitions of unsaturated flow porosity(Richards eqn),Mass transport porosity

I am running a 2D vertical cross-section model with Richards equation for flow and mass transport for 2 species. There are three porosities: 1. Unsaturated flow porosity, which I assume is Specific Yield 2. For Mass transport 1 there is a porosity option and 3. for For Mass transport 1 there is another porosity option.

How each species can have different porosities? Is it physically possible?

There are special cases where different porosities (or other mass-transport properties) for different species may be useful. In general, they should be equal. Unsaturated porosity (total porosity), though, can be different from porosity for mass transport, as not all pores are necessarily accessible to flow and thus mass transport.