According to the docs in /libs/granite/ui/components/foundation/form/multifield/render.jsp there is the possibility to have several subcomponents within a multifield:
...
* If the subcomponent is a composite, it is its responsibility to concatenate the fields into
* one, before the form gets submitted, and to separate them again on rendering
....
Does someone have an example for such a multifield with a composite children? The only references to multifield I found in AEM6 were with only one simple component as a child.