A GSTIN (GST Identification Number) is a 15-character alphanumeric code assigned to every registered taxpayer. The first two digits are the state code, the next ten are the PAN of the business, and the remaining digits encode the entity number within the state and a check character.
Because part of the GSTIN is the state code, a business registered in multiple states has a separate GSTIN for each. Validating a customer or vendor GSTIN before billing helps avoid input-tax-credit problems later.
The tail of the number carries meaning too. The thirteenth character shows how many registrations the same PAN holds within that state — it runs 1 to 9 and then A to Z — the fourteenth is presently 'Z' by default, and the fifteenth is a check character computed from the rest. That checksum lets software reject mistyped GSTINs instantly, which is why a well-built billing screen can flag a transposed digit before an invoice is ever saved.
A structurally valid GSTIN is not necessarily a usable one. The public search facility on the GST portal reveals the legal name, trade name, registration date, taxpayer type and — critically — whether the registration is active, cancelled or suspended. Checking new customers and vendors there, and re-checking periodically, matters because supplies billed to a cancelled GSTIN and purchases from one both create problems that surface months later in return matching.
Day to day, the GSTIN threads through everything: it must be displayed at the registered premises, printed on every tax invoice, and quoted invoice-by-invoice in the B2B tables of GSTR-1. A single wrong character there routes the buyer's input tax credit to a stranger's GSTR-2B, and the fix — an amendment in a later return — costs the buyer a wait for credit. Branch billing is the other trap: each state's supplies must go out under that state's own GSTIN, not head office's.