The index of the item in the parent field's initial value (not necessarily the index where the dependent form
was initially rendered: If the item is a new item, then initialIndex is null).
The unique identifier for the item. For rendering purposes.
A form state that is part of another form state.
SubFormStateis a trimmed-down version of a whole form state that stores only the data and none of the higher-level functionality. Omits certain data that are either global (e.g.formType) or contained in another value (e.g.formValues).