XSLT and advanced mapping
A graphical mapping model is translated to XSLT. XSLT is a language to transform XML documents to other XML documents. From the graphical model, XSLT 2.0 compliant XSLT is created.
An alternative of using the graphical editor is using XSLT directly. This is called advanced mapping. You can use a hand-written XSLT (or an XSLT created using external tooling). Or you can start with the graphical editor and use the generated XSLT as a basis for further editing.