Pieces

Pieces are an ideal choice for structures that may repeat across product, document, and folder shapes, and can also facilitate subtype and ad hoc polymorphism within your content modeling. Pieces are defined once, then added to other shapes or to structural components such as chunks and choices. Pieces also have a nesting ability similar to that of structural components. 

Pieces come with a powerful nesting capability. You can add pieces to other pieces and structural components like chunks and choices to create sophisticated multi-level structures. Currently, three levels of nesting are supported. The components on the third level of nesting may only contain non-structural components.