The framework includes two catalogs:
The modules in this catalog provide implementations of various value types, for example Markdown. They are intended to be consumed by your application.
Each will have an applib submodule (for use within the domain logic) as well as various extension modules to be made available to the framework itself (to teach it how to persist or render the value type).
These modules in this catalog extend the capabilities of the framework itself. They are generally not consumed directly by your application. For example, the "Excel Download" extension allows collections to be downloaded as an Excel
.xlsxspreadsheet in the Wicket viewer.
That said, some extensions do also provide an applib. For example the gmap3 extension provides an applib so that domain objects can expose their location (typically a Java interface to be implemented); this allows the extension to plot them on a map.