Boundary conditions (loads and restraints) are applied to surfaces and meshes that correspond to boundaries of the solids on which simulation is being carried out. Grasshopper will allow b-reps to be decomposed into their constituent faces to which boundary conditions can be applied, and there may be situations where this is desireable for automation. However, for manual application of boundary conditions to existing solids, Rhino's ExtractSrf command with Copy set to “Yes” is a straightforward mechanism to specify faces interactively.