GSTIN

Because India taxes supply at the state level under a national framework, so a business operating across states is, for tax purposes, several taxpayers wearing one name.

The GSTIN is the unit of tax identity, and getting its place in the data model right decides whether compliance is a report or a monthly reconstruction. It is the concept global ERP documentation is least helpful on, because most systems assume one tax registration per legal entity — an assumption that is simply false in India and quietly wrong in any implementation built on it.

• A GSTIN is per state, not per company. One legal entity operating in four states holds four GSTINs.
• Each GSTIN files its own returns, holds its own input tax credit, and reconciles independently — credit does not pool across registrations.
• The first two characters are the state code and characters 3–12 are the entity's PAN, so a GSTIN and a PAN are structurally related but not interchangeable.
• A transaction's place of supply, not the seller's address, determines whether CGST+SGST or IGST applies.

The structure is chosen at kickoff, before anyone has filed a return through the system, and by the time the first filing exposes it there are months of transactions on the wrong shape. It is the one modelling decision in an Indian implementation that is genuinely expensive to reverse, which is why it belongs in discovery rather than configuration.

• SEZ units, where supplies are zero-rated and the normal intra-state logic does not apply.
• Composition-scheme suppliers, who cannot pass on input credit — buying from them changes your effective cost, not just your paperwork.
• Reverse charge, where the buyer remits tax the supplier would normally collect.
• Bill-to and ship-to in different states, where place of supply follows a specific statutory rule rather than intuition.
• A customer whose GSTIN is cancelled mid-relationship, invalidating credit on invoices already booked.