Also called POS under GST, place of supply rules
Place of supply is the place where GST law treats a sale as happening. Comparing it with the seller's location tells you which tax to charge. Same state means CGST and SGST. Different state means IGST. It matters for every invoice, because the place of supply decides which government receives the tax.
For goods that are moved, the place of supply is usually where the movement ends. For goods sold over the counter, it is the shop's location, even if the customer lives in another state. Getting it wrong sends tax to the wrong government, and fixing it later is slow.
A shop in Bengaluru ships an order to Chennai. The movement ends in Tamil Nadu, so it charges IGST. A tourist from Chennai buys the same item at the Bengaluru counter and carries it home. That is a sale in Karnataka, with CGST and SGST.