Multi-instantiated Fields and persistance Id
1) The persistenceId is a generic field for any business object. It is unique per business object and automatically generated per the engine. You need to use it for any operation on a business object instance that is not a creation. 2) Not sure about this one
so I can discard it, if I have any other source of identification and also does not plan to update, modify or delete an item in further tasks? Nice to know.
2.) This one was fairly easy to figure, it is just an arry you can traverse like any other array and it is located in your data structure within $data