If you apply a modifier to the Existing model then it is by default applied to the Proposed model as well, because you have altered the terrain.    The idea is that the Existing model represents the existing terrain + is derived solely from your source data + has no modifications applied to it. I think. All the modifiers are then applied to the Proposed model + this way the Cut + Fill calculations correctly reflect the changes to the terrain between the unmodified existing state + modi
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