A delivery challan goes with goods that are moved without a sale being made at that moment: job work, goods sent on approval, or a delivery that will be invoiced later. It lists what was sent and how much.
Rule 55 of the CGST Rules lists the cases: goods sent for job work, goods sent on approval, liquid gas where the quantity is known only on delivery, and some others. A challan is also common for everyday deliveries where the invoice follows later.
The challan is not a tax invoice. When the sale happens, use “Make an invoice” to copy the delivered items into an invoice and add rates and GST.