Information about your production asset and its various Assembly representations is stored in an assembly definition (assemblyDefinition node). Assembly definitions let you switch between the different representations of the production asset. A Maya scene can contain only one assemblyDefinition node, so each assembly definition must be as saved as a separate .ma or .mb scene file. (See Create an assembly definition.)
When you create an assemblyDefinition node, it appears in the Outliner, Node Editor, and Hypergraph represented by this icon: 
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The following image illustrates a typical assembly definition.
When using assembly definitions in your scene, consider the following:
