The last character is a mod-36 checksum over the first fourteen. Change one character anywhere in the number and it almost always stops matching — which is how a mistyped GSTIN gets caught at the counter instead of in a return three weeks later.
Many free "GST number validators" only check the length and the pattern, which happily passes a number that can never exist. Our GST number checker computes the check digit.
That the number is well formed — not that it is registered, active, or belongs to the business in front of you. There is no public API that answers that, and any tool implying otherwise is guessing. For the registered name and status, look the number up on the GST portal.
A customer's GSTIN decides whether the invoice is B2B, whether it appears in the B2B table of your GSTR-1, and — through the state code — which tax you charge. Get it wrong and your customer can lose the input credit on it.
Not from any public API — only on the GST portal. A format checker can tell you the number is well formed and free of typos, which is what you need before it goes on an invoice.
A mod-36 check digit calculated from the first fourteen characters. It is what catches a mistyped number.
Yes — characters 3 to 12 are the holder's PAN, unchanged.
Aned Book works the tax out from the bill you are raising, so none of the above is a decision anybody has to make at a busy counter.