Region curves vs extruded curve

It's a common thought that the Region curves function is a "hard" boundary that the cutter can't leave. This is a negative assumption. The region curve tells the tool path generator where to cut the model, it doesn't fence the cutter in that area. There is a simple way to make a fenced area with an extruded curve, for example the region curve itself. This will cause the cutter to stay inside the region. Here is a simple example:

Tip! Make a region curve, expand it with a bit more than the cutter radius. Make an extruded curve with the new expanded curve and then use the the smaller curve for the region. Doing it this way the tool path will not climb the wall, it will go directly to the model surface. Here is a bit more sophisticated version, yet simple though: