Amex allows users to create additional cards for their friends and family. These are called supplementary or additional cards.
A supplementary card is an additional Card that lets you share the benefits and convenience of your American Express Card with family or friends.
It can be compared to a joint account where all the cards are linked to the same source but, unlike a joint account, supplementary cardholders may not have the same access as the main account holder.
The transactions made on these cards show up under the main card's transaction and the balance on them is the same as the balance on the main card: this means that from the main card is it possible to retrieve all the transactions done through the main card and the supplementary ones.
When consenting on AMEX's website only the card the user consents for is shown and when the
/cardsendpoint is called, TrueLayer will only return that card.
The only way for getting the account_id of a supplementary card is by connecting a known supplementary card.
Amex doesn't return balance for supplementary cards, so any calls to the
/balance endpoint will receive an access_denied error.
This is a design decision made by the provider. The rest of the calls will work as expected.