How to interpret boundary conditions for matlab?

I know that these are called Danckwerts Boundary conditions but I still don't understand.

Uc = Uc(x=0) - D*dc/dx (x=0)

dc/dx (x=L) = 0

Which one would be my lower boundary and which one is the upper boundary? Do I have to switch anything around?

My first assumption would be that the one with x=0 would be my lower and the one with x=L is my upper boundary. L in this case is equal to 100. But I just want to make sure. I am also given values for D, U, & c which are 5000, 100, and 100, respectively.