I was testing the POS page in offline mode. I made a POS invoice for an Item, selected a Batch from the modal popup and made the payment.
I found out that the Batch I selected here, has no effect on the actual Sales Invoice that gets synced to the server. It seems the synced Sales Invoice items have the Batch selected based on the standard ERPNext rule of preferring Items expiry first, which could happen to be a different Batch from the one selected on the POS page.
sales_invoice_doc in LocalStorage and did not find mention of the Batch in the