Middlegen is an xdoclet generator. It doesn’t generate all classes itself, it creates just those with the xdoclet params (just like your wizzards do) but for more. e.g. CMR between several tables, JDO classes. Afther that, xdoclet takes over and generates all classes.
Just look at it as a visual CMR/CMP/JDO/…. wizard. One has to know even less about xdoclet tags when using middlegen.