An ICCID (19-20 digits) is a unique identifying number specific to a SIM card. Both physical and eSIMs have an ICCID number recorded in Dashboard. The ICCID number is used to assign a SIM card to a device.
For end users with a physical SIM, the ICCID is visible in 2 locations:
On the back of the larger plastic card that enclosed the SIM card under the bar code.
On the SIM card itself.
With an eSIM plan, the end user can use their activation code or QR code to configure their plan.