GSTIN (GST Identification Number)

In One Line

The 15-digit unique number assigned to every GST-registered business — your GST identity on every invoice, return, and filing.

The first two digits are your state code, the next ten are your PAN, followed by a few digits identifying the registration within that PAN and state. You'll need it on every invoice you issue, and marketplaces won't activate a seller account without a valid one on file.