Mr. Joshi, for passenger transportation, Please refer to provisions as stated in Sec 12(9) and Sec 13(10) of IGST Act. Sec 12 is applicable if both service receiver / provider based in India or else Sec 13(10) is applicable. K3 code is single tax code used by Airlines. Indian Airlines are stating IGST or CGST+SGST on Invoice for domestic transportation and IATA uses K3 code in its Invoices. Furthermore, you will get the ITC reflected in your GSTR-2A/2.
Sec 12(9) of IGST Act: The place of supply of passenger transportation service to,-
(a) a registered person, shall be the location of such person;
(b) a person other than a registered person, shall be the place where the passenger embarks on the conveyance for a continuous journey:
Provided that where the right to passage is given for future use and the point of embarkation is not known at the time of issue of right to passage, the place of supply of such service shall be determined in accordance with the provisions of sub-section (2).
Explanation.––For the purposes of this sub-section, the return journey shall be treated as a separate journey, even if the right to passage for onward and return journey is issued at the same time.
Sec 13(10) of IGST Act : The place of supply in respect of passenger transportation services shall be the place where the passenger embarks on the conveyance for a continuous journey.