Inter-State supply classification determines whether IGST or CGST and SGST apply based on supplier and place of supply. Determination of whether a supply is an inter-State supply depends on the locations of the supplier and the place of supply; supplies across different States or Union territories, imports until crossing customs frontiers, and supplies from India to outside India (including to or by SEZ developers/units) are treated as inter State. Place of supply rules govern tax treatment; invoicing must state place of supply and State to secure destination based tax allocation, with noncompliance attracting penal provisions.