An APL package is a container for layouts, resources, and styles that you can import into your APL documents.
An APL package is a single JSON file that follows the APL document structure. The package doesn't use the
mainTemplate property, although including this property doesn't invalidate the package.
The Alexa Design System for APL provides a set of packages you can use for styles and pre-built layouts. See Alexa Design System for APL