A GSTIN looks like a reference number, so it gets typed the way reference numbers get typed — quickly, once, and rarely checked again. It is not a reference number. It is an instruction, and it is the one your billing software reads before deciding what tax to charge.
The first two digits are a state
Every GSTIN begins with a two-digit state code. That is the part your software uses. Your own GSTIN says where you supply from; your customer's says where you supply to. Those two together decide the shape of the tax on the bill.
Same state or different state
When both parties are in the same state the tax splits into two halves. When they are in different states it does not split. Software that knows both GSTINs never has to ask you which — and should never ask you which, because the moment it does, it has handed you a decision you can get wrong.
Where careless entry actually hurts
- A wrong state code puts the right total under the wrong heads, which is the kind of error that survives to your return.
- A customer saved with no GSTIN is treated as unregistered, and that changes how the supply is classified.
- A typo in the middle of a GSTIN often still looks plausible, so nothing complains until a return does.
Two habits worth having
Check the GSTIN against a document rather than a WhatsApp message, and let the software fill the state from it rather than choosing the state yourself. If the state field fills itself and matches what you expected, the GSTIN is at least internally consistent. If it fills something surprising, you have caught the problem at entry rather than at filing.
How BizGST Pro handles it
The state fills itself from the GSTIN on both your business and each customer, and the CGST/SGST/IGST split on every invoice is derived from that pair rather than asked as a question. See it for yourself on the free GST calculator at bizgstpro.com/tools/gst-calculator — switching between same state and another state changes the breakup in front of you.