Where are you wanting to save the List's data, to the local database or an external database?

If you POST the data to a REST service, in either case, the only difference is where you are POSTing the data.

If data is stored locally, you would have to write an Activity yourself that checks for inserts vs updates by checking for matching keys in the database.

If fewer rows exist in the POST than exist in the database,  you must delete the rows in the database where the key(s) not match any of the key(s) in the POST.

But to know what rows to update or delete, there has to be something that is referenced. In Pega this is what a Declare Index does, namely it stores rows that reference something. In all likelihood that "something" is a case, but does not have to be.