ICCID (Integrated Circuit Card Identifier)
Updated
The ICCID is the serial number of a SIM card, usually 19 or 20 digits, printed on the card and stored on the chip. Its structure (industry code, country code, issuer, serial number, check digit) follows ITU-T recommendation E.118. It identifies the card, not the subscription.
In practice
The ICCID is the number you use to activate, suspend or top up a SIM in your account or through the API, because it is printed on the card before any network registration.