just so that I understand the situation, can you tell me if the following is correct: 

1. You have a Repeating Section on a Classic Form, somewhere (I assume a list item), which gets saved with some arbitrary data on it.
    
2. When the Form is Submitted, the Repeating Section XML is captured, and each Repeater Row is used to generate an Item on a List called “Master”
    
3. Now, when those Master Items have finished processing, you’d like for the data saved to them to be pushed back to the original Repeating Section Item that was used to generate them

Does that correctly outline what it is you’re trying to do? 

If so, then I have further questions:

1. How does an Item that has been generated on the Master List from a row of Repeating Data know which Repeating Section Item it came from? Better yet, is there any association between the Repeater Row and the subsequent Master Item?
    
2. You say that you want users to be able to select which items get pushed back into a Repeating Section. Is there a way that you’re already identifying which Items *would* go back vs. which ones wouldn’t?

Depending on the granular details of what you want to do, this can be trivial or fairly advanced. Getting a bunch of items to shake down into a single item is a sort of blind spot within Nintex as far as I’m concerned. That said, I have created a fairly robust way of doing it out of box, but I’ve never tested it beyond my own systems and it’s not a straight forward way of designing a workflow. Nevertheless, I should at least be able to provide some sort of answer. 

Let me know! 

Hi @MegaJerk , thanks for your reply, the following is answers to your questions:

1. the Master list just list down the individual repeating data from the original form, there is no relationship
2. the users can select the Master list items by ticking the check box next to the item so as to merge them back to a new original form. This will allow more flexibility for the user to select and choose which items from the master list to reuse.

Unfortunately checking the checkmark on multiple list items will not allow you to run a workflow on those items at the same time or in any batched way. I believe the way it works is you can check any number of them, but when you right click → Advanced → Workflow, it will only take you to the Workflow properties of whichever item your mouse cursor was over when you right clicked on it. 

So you’d have to have something in place on the Master List Items that indicates how any given item is associated to any *other* item in the same list, and then you’d also need something in the workflow that runs on each one of those items which would allow your users to flag when, whatever work is being done to them is finished, whether or not they should be pushed into some other condensed Form elsewhere.