As described in Product Overview, the Internationalization Module is built on the Unicode® Consortium International Components for Unicode (ICU), the industry standard for Unicode processing.

ICU constructs used by classes in the Internationalization Module are protected members of those classes. Should you need to access the underlying ICU constructs — to call obscure ICU methods, for example, or to interact with preexisting ICU code — you can do so by deriving from the appropriate class in the Internationalization Module. However, be aware that the non-public API may change without warning in future versions of the Internationalization Module.