An add-on, also referred to as an "extension," is intended to improve or expand, for example, sewing patterns.
To illustrate this, you can take a look at the Girly Sweater e-book by Schleiferlwerk. By default, this e-book does not include a hood, top, or fake cord. However, the add-on for the Girly Sweater offers the possibility to add a hood. It is important to carefully read the description, as the add-ons may not be intended for all garments.