This is a highly specialized API that probably only very few people with highly specific needs will need to use. Basically, it lets you add, remove or change layout and logical properties of sections of inline content at the level below par elements during creation of the internal, unified document tree that is used by subsequent export filters as their basis.
If you feel you need to use that API to solve a problem you are facing, we highly recommend to contact us in advance at <support@infinity-loop.de> and discuss the issue with our technical experts. We will send or point you to up-to-date API documentation and help you regarding your implementation questions and needs as much as we can.