This is a regular Fuel Card, not a brand-specific voucher.
All fuel stations use point of sale devices registered for fuel purchases, as required by the banks. The Fuel Card is only eligible for use on those devices, normally the ones which are managed by the petrol attendants.
Thus the card is not linked to any brand but rather restricted for use at these point of sale machines which are standard across all fuel stations.
This is also the reason why the card will not work in the shop or for other products such as gas refills - the shop POS device is not registered for the sale of fuel