A component definition is automatically registered when you create or add a component using:
A component definition contains the geometry and attachments of the component. Attachments are automatically generated from the geometry based on the Auto attachment factor (defined on the General page of the Assembly Options dialog), and stored with the component definition. This factor is applied to the bounding box of the geometry of the component to create a new bounding box. Any attachments that lie within this bounding box are automatically included in the component definition.
Component definitions act like templates, and are used to create instances of a component. Nearly all assembly operations are performed on the components themselves. Usually, component definitions are not visible to you, but there are two areas where you will use component definitions: