Let me give you one scenario (This is the practical thing which I have experienced)
If 'manish' resides in Maharashtra having a commercial property in maharashtra and registered in GST with maharashtra registration. Renting the property to Arun, who resides in West Bengal with registered No. of west Bengal.
What Manish has to charge in his invoice? CGSt-SGST or IGST? Manish stay in Maharashtra and having a commercial property in Maharashtra only.