Included with the framework are several catalogues/libraries.
These libraries are intended to be used by your application:
Libraries and utilities to be called by the domain logic of the Apache Isis application itself.
Implementations of various value types, eg Markdown. 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 libraries are used to integrate between apps, acting as the glue code. Some are implementations of SPIs defined by the framework (for example to publish events), others are for use by your application (for example to make REST calls):
Libraries and utilities for bounded context (BC) mappings. In other words, tools to help integrating system together.
These libraries extend the capabilities of the framework itself:
Describes a number of extensions to the various components that make up the framework.
For example, the "Excel Download" extension allows collections to be downloaded as an Excel
.xlsx spreadsheet in the Wicket viewer.