Skeleton Model
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A model that contains a sketch, surface and/or solid model used to facilitate top-down modeling, by allowing multiple users to access interface geometry from a single model. When the skeleton model is changed, those changes are proliferated to any associated geometry in other models that use the skeleton reference geometry. A skeleton model will always have ZERO MASS, as it is a reference-only model.
Alternate Definitions:

PTC - A predetermined structural framework of a component. You create it to define space, geometry, component placement, and connections and mechanisms at the beginning of the design process. With a skeleton model, you can pass along essential design information from one subsystem or assembly to another.