Place of Supply is the GST framework's way of allocating tax revenue between states. For goods: usually the destination state (where shipped). For services: typically the recipient's location, with exceptions (passenger transport, accommodation, restaurant — supplier's location).
Intra-state: place of supply = supplier's state → CGST + SGST. Inter-state: place of supply ≠ supplier's state → IGST. Wrong place-of-supply = wrong tax type collected = customer can't claim ITC + supplier may face audit.
For Indian D2C ecom shipping pan-India, place-of-supply is determined per order (shipping address, not billing). Most order-management systems get this right; manual invoicing is where errors creep in.