A Machining Allowance is necessary if the optimisation result is manufactured with Additive Manufacturing processes where functional surfaces (Non-Design Spaces) are post processed. With MSC Apex Generative Design Machining Allowances can applied on surfaces where a specified amount of additional material will be included to later be removed in a precision machining process like milling.
The optimisation produces different outputs for different requirements of the model - once a print geometry and once a nominal geometry.
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The Machining Allowance needs to be added to all functional surfaces. So the Machining Allowance should be applied in combination with the Non-Design Spaces. Touching faces should always be selected in one Machining Allowance because only then the Machining Allowance is one connected volume instead of just several offsets of the faces. The picture below shows the difference of the selections:
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The optimisation is carried out and calculated without taking the Machining Allowance into consideration. It is added after the calculation process is done but before any smoothing algorithm has run over the geometry to guarantee a design without any transition problems between these different geometries.