The application is - Moisture transfer from one phase (consisting of solids, air and water vapour) to another phase (consisting of air and water vapour). To this end, there must be porosity in the 1st phase, and there is no need for porosity in the 2nd phase.
The solids in phase 1 are basically separate boxes, so each box is considered a cell zone containing phase 1. Phase 2 (air and water vapour) surrounds these boxes. The two domains are phase 1 and phase 2, between which there is mass transfer.