Analogously to the situation for the volume model equations, a generalized
formulation of models describing the flux of quantities across the
interface between two adjacent segments has been discretized, leaving the
user the possibility to provide models determining the coefficients of the
underlying equation system. In the following equations 
denotes the
flux of the quantity 
leaving segment 
whereas 
gives the flux
of quantity
entering segment 
(Fig. 4.1).
The default boundary condition (4.6) for all quantities is a homogeneous Neumann type condition which defines a zero flux over the boundary.
These default boundary conditions can be replaced for any combination and number of quantities on both of the adjoining segments by either defining the flux over the interface (Section 4.2.2.1) or defining the actual concentration of the respective quantities by adding Dirichlet type boundary equations as described in Section 4.2.2.2.
