This is a highly specialized API that probably only very few people with highly specific needs will need to use. Basically, it lets you set the points in the document where (nesting) section breaks should occur, based on custom criteria beyond the usual class attribute of the respective paragraph. For example, you might base your decision on the text content of the element.
To implement, you need to subclass de.infinityloop.upcast.treeprocess.SectionProcessorBase and implement checkParagraph(). See the API documentation for more details.
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.