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Interfaces are all functional faces of a model so all faces where boundary conditions are applied to. With the Interface Tool these faces are selected to create both Non-Design Spaces and Machining Allowances.
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The Non-Design Space contains the necessary geometries of the part. Any part for which you need a clearly defined geometry should be a Non-Design Space. Non-Design Spaces can be created as a part of an interface or as an own solid/geometry which are covered from the Design Space.
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