Sub Form Field function is introduced in Joomla! 3.6. This function allows developer to nest XML forms inside each other or reuse existing forms inside the current form.
The field allow to include any existing form into the current form. If attribute multiple set to true then the included form will be repeatable. The Field have a couple "predefined" layouts for display the subform as table or as div container, and of course it allow to use your own layout. Field support Default values from the included form, and from json string in default attribute. Last have higher priority.