A "conductance" used to enforce specified concentration/temperature boundary conditions. This parameter is set in dataset 5 of the main input (".inp") file. GNUU works analogously to GNUP; see Section 3.5 and Appendix B of the SUTRA documentation, and " Q: What do the boundary conductances GNUP and GNUU do, and why are they important?" on the " Frequently asked questions" page.