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The Section, Block and Paragraph elements are part of RadDocument's structure that is built as a flow document. The hierarchy you found in our documentation is indeed placed there as a reference when building a document from code. Extending it is possible, but I don’t think it is necessary in your scenario, as the operations you have listed could be achieved with the document elements as they are.
Generally speaking, RadDocument exposes its Sections through a property (RadDocument.
Sections), each Section – its Blocks (Section.
Blocks), a Paragraph – Paragraph.
Inlines, etc. You could use these collections to determine what to select and then implement cut
/paste functionality of the Selection
When it comes to providing a UI for that, you can try implementing UI layers such as the TableMovementUILayer. Most visual representations in the control's code are built in that way. I suggest you refer to this article for more information.
I hope this helps!