After reading this document, you should have an idea about, what OOUIs are, and the interfaces, which can be used between the OOUI Kit and the user application.
Guidelines for client and server implemention give the books Berlin OOUI Kit Client Guideline (DocBook-ID berlin-ooui-clienttut) respective Berlin OOUI Kit Implementation Guideline (DocBook-ID berlin-ooui-servertut) from the same source, both basing on the book you are reading at the moment.
The Berlin OOUI Kit Implementation Guideline, Chapter Realizability especially discusses the question, if and how an implementation of an OOUI Kit is at all possible.
This book is no formal specification. Such a document will follow at a later time.