'Setting In' - defining the outline. Since we will be under- and backcutting, we only need to set in down to about 1/8in. 3mm.
Before you you set in the edges make sure you have finished modelling the surface to a final finish. When you set in, this is what is assumed: you bring the form down to what will be the edge, then set in.
There's the rule: Finish the surface out to what will be the edge. Then set in the edge.