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It is possible to take solids as Retained Volumes into consideration during an optimisation. Retained Volumes are not part of the Design Space and are not to be optimised during the optimisation - so the geometry is not changed. However, applied loads and boundary conditions will be taken into consideration or they can be used to transfer loads from one part of the Design Space to another. To take Retained Volumes into consideration, the Generative Design Configuration Tool needs to be used. Pleas Please have a look for further information here and for an example tutorial here.
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