Substructures are collections of elements that have been grouped
together, so the internal degrees of freedom have been eliminated for the
analysis.
Using a substructure makes model definition easier and analysis
faster when you analyze a model that contains identical pieces that appear
multiple times (such as the teeth of a gear), because you can use a
substructure repeatedly in a model. Substructures are connected to the rest of
the model by the retained degrees of freedom at the retained nodes. Factors
that determine how many and which nodes and degrees of freedom should be
retained are discussed in
Generating substructures.
Substructure definition in your model follows two sets of steps.