A customer record looks like an address book entry — name, phone, address, done. Then one field on it turns out to have been deciding the tax treatment of every invoice to that party for the last year, and nobody knew.
The field that does the work
The GSTIN. Not because it is printed on the invoice, though it is, but because its first two digits are the state code. Your state and your customer's state are what determine whether a supply is treated as within-state or between-states, and therefore how the tax on the invoice is split.
You do not choose that split. It follows. Which is exactly why a customer created in a hurry with the GSTIN left blank causes trouble months later — not immediately, and not visibly.
The three ways this goes wrong
- **Blank GSTIN on a registered customer.** The invoice goes out without it, and your customer's ability to claim credit is affected. They will notice before you do.
- **A GSTIN typed with a transposition.** It looks right at a glance and the state code may still be plausible, so nothing complains. Every invoice to that party then carries a wrong number.
- **A GSTIN belonging to a different branch.** Large customers have one per state. Bill the Maharashtra GSTIN for goods delivered in Gujarat and the paperwork disagrees with the lorry.
The address is not decoration either
Place of supply is a real field on an invoice and it is not always the billing address. Getting the address right at master-creation time is what lets everything downstream be right by default, rather than corrected one invoice at a time.
Why importing masters is better than typing them
Not speed — accuracy. A GSTIN typed six hundred times by a person will contain errors. A GSTIN copied from a file that already had it will not. If the source data is untidy, an import that leaves a bad value blank is doing you a favour: an empty field is visible, and a wrong one is not.
How this looks in BizGST Pro
The customer record carries the GSTIN, the state code and the address, and the invoice screen reads them: pick the customer, and whether the tax shows as CGST and SGST or as IGST is settled before you have typed a single line. The Tally import fills these in from your existing masters, derives the state from a valid GSTIN, falls back to the state name when there is no GSTIN, and stores nothing where the value did not look like a GSTIN at all.
The five-minute job worth doing
Open your customer list and look at the GSTIN column. Not to fix everything — just to see how many are empty. That number is how many invoices are currently being decided by a fallback rather than by data, and it is usually larger than people expect.
What this article does not cover
Which supplies are treated which way, what must appear on an invoice, or any rate or threshold. Those come from the GST portal and your accountant, and we do not state them from memory. What is above is about the record being complete, which is your side of it either way.