With the Advanced User Settings the automatic Non-Design growth can be deactivated. Per default it is activated, thus all Non-Design Spaces grow a little bit beyond their boundaries to ensure a trouble-free and clean intersection. This is done with an automatic detection of Non-Design faces. Sometimes this can lead to issues.
If Machining Allowances are applied, the automatic Non-Design growth can disabled. The Machining Allowance ensures also a trouble-free intersection to create all the needed functional surfaces.
These two options create different transitions between the Non-Design Spaces and the overall Design structure.
The disabled Non-Design growth creates a smooth transition between the Non-Design Space and design structure. Nevertheless, in this area stress risings can appear. The Post Processing with a machining process like milling is much easier because no unintentional cuts into the design structure can happen.
The enabled Non-Design growth (default) creates a hard transition with a notch to the design structure. This transition leads to stress risings at the notches which influences the fatigue life of the part. Furthermore, the Post Processing after the manufacturing of the part with e.g. milling is quite difficult. The position of the notch should not be moved any further in Design direction.
It is not quite clear which transition has a better impact on the fatigue life of a part and has to be evaluated by the engineer on for each case.