As for what a core boundary actually means, it's that point along a wall's thickness, where the main make-up joins the finish. For example, if I had a 100mm block wall plastered on both sides, the plasterwork would be outside the core, while the blockwork (the main part of the wall) would be inside the core. It's useful to have a differentiation between the 2. If for example you were dimensioning a room, normally you would want the dimensions to snap to the blockwork on the wall, rather than the finished face, . . . .