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The Access Region Tool is responsible for creating placeholders for void spaces which are necessary for installation and maintenance reasons. Further Information here.
The Clearance Region Tool is responsible for creating void spaces around parts which are part of the assembly but are not part of the optimisation model. Further information here.
The Machining Allowance Tool is of course also used in these optimisation models to produce ready to manufacture results. Further information regarding the Machining Allowance Tool here.