Question

in partial differential equation, for a boundary conditions, there are three or more types of it. such as Dirichlet, Neuman, Mixed , robin, etc.
For a Mixed(or not, correct me if im wrong), how to determine the coefficient value for

A u + B u' = C
where A, B, and C are constant,
u is some function,
u' is the first derivatives with respect to normal(first gradient)

thanks guys for ur help b4