When adding a Mass Source node in the coupled Phase Transport interface, it is by default assumed that the net mass source of all phases, Qtot, equals zero. This implies

However, when mass is transferred from or to phases not accounted for in the phase transport interface — for instance, when mass is released or absorbed by the solid part of the porous matrix — select the Mass transfer to other phases checkbox. In this case the mass source for the phase calculated from the volume constraint should also be specified, and the net total mass source Qtot is supplied as a right-hand side to the Darcy Equation 8-23 for .

Similarly, when a Boundary Mass Source node is added in the coupled Phase Transport interface, the sum of the specified mass fluxes for all phases is supplied as a mass flux boundary condition for the Darcy Equation 8-23.