The water rate boundary condition in Seep/w refers to
When no boundary conditions are applied to the edge of a SEEP/W domain, a no flow (Q=0) boundary condition is implicitly applied by the solver. There will be no outflow through the edge of the domain, unless a boundary condition is purposely applied by the user.
A potential drainage face boundary condition is a no flow (Q=0) boundary condition in which we activate the "seepage face review" toggle to allow the solver to choose whether flow should or should not be allowed through the location where the boundary condition was applied.
Please review SEEP/W's reference book to learn more about the various types of boundary conditions:
https://downloads.geoslope.com/geostudioresources/books/11/4/Heat and Mass Transfer Modeling.pdf