Included with the framework are several catalogues/libraries:
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).
Libraries and utilities for bounded context (BC) mappings. In other words, tools to help integrating system together.
There are two further catalogues: