When profile machining you can have a negative offset. This enables you to place the tool over the surface edge and therefore to produce a profile toolpath along a curve. The following example uses a 20 mm End mill and uses a Lower limit base position of Drive curve.
Side of Outside edge, Radial offset of 0.
Side of Outside edge, Radial offset of -10 or, Side of Inside edge, Radial offset of -10.
Side of Inside edge, Radial offset of 0.
Looking in detail, where the yellow circle represents the cross section of the tool:
