It’s another way of stating ghost points. See the definition here:
http://diffeqoperators.sciml.ai/dev/operators/derivative_operators/#Derivative-Operators
what it amounts to is that you only need the H_buff[2:end - 1,2:end - 1] .= max.(0.0, inn(H_buff) .+ dHdt) equation, in which case you don’t need the buffer because you can instead just return max.(0.0, H .+ dHdt)