As opposed to the other methods for reaction force computation, the boundary flux based tractions are computed not only on external boundaries, but also on interior boundaries. On interior boundaries, there are then two traction fields: One acting from each of the domains sharing the boundary. These internal traction fields are contained in the variables interface.iTracux and interface.iTracdx. The letters u and d in the variable names indicate the upside and downside of the boundary, respectively. If you need the value of the total force acting on an internal section through your model, these variables can be integrated. The interface.iTracux and interface.iTracdx variables are only available if the Compute boundary fluxes check box is selected in the Discretization section of the physics interface, and there are interior boundaries in your model.