This 125kHz ISO standard proximity card is 1.8mm thick and can have printed material attached to the front. It is supplied with a pre-assigned unique code.
Proximity Card Technology
Proximity cards (or access cards) are hybrid technology that incorporate a simple electric circuit (proximity tag and coil) imbedded in a plastic card. No battery is needed and the circuit is not removable without destroying it and the card. When the card is correctly positioned with respect to a reader a high speed RF signal passes between card and reader and unique data is transferred.