The word mixin has both a general and more specific meaning.
In general, designs that are inherited by other designs are also known as mixins, as the rules from the inherited designs are “mixed into” the inheriting design.
More specifically, the word mixin refers to designs that are meant only to be components of the inheriting design. That is, they are non-instantiating designs, intended to provide a collection of rules to the inheriting design.
Best Practice: Include the word "mixin" in the design name when a design is intended only to be used as a component (not instantiated as a part).
Design LegAssembly : AssemblyComponents BOMItemMixin IvAssemblyDocument