How to identify fulfillment line number in fbdi tempate for update

Summary:

The Sales order line is split by GOP and we have requirement to update the fulfilment line 2 for quantity.

Please suggest how to specify the fulfillment line number in FBDI for update.

e.g. the order has split line 11-1 and 11-2 and want to update 11-2 for Quantity though FBDI

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