Help understanding how boundary conditions for identity pairs work

Hello everyone,

I’ve been reading through these discussions and the Comsol help guide, but am still having trouble wrapping my head around exactly how the boundary conditions for identity pairs work – apologies if the answer to my question has been answered already and I’ve just overlooked it.

I’m mainly confused about why there are so many options for the pairs boundary conditions when they just seem to be overridden by the “no flow” conditions that accompany them. No matter what pairs BC I select, I feel like I must right click on it and add additional boundary conditions to override these “no flow” BC’s. Anything I enter in that original pairs BC node seems pointless then, because it seems like these additional boundary conditions must be what truly control the boundary.

Can anyone explain the role the main pairs BC node plays in defining the boundary condition, as compared to the no flow BC that accompanies it and the other BCs you can define by right clicking on the main pairs BC node? In the Richard’s Equation physics I work with, the options for this main pairs node I keep referring to are pressure, pervious layer, flux discontinuity, fracture flow, or continuity.

Thank you!!