The difference between the primary card and the secondary card of a credit card (debit card) is that the primary card can return the bill amount of the primary card plus the secondary card, while the secondary card can only return the bill amount of the secondary card. Even, some bank daughter cards cannot be repaid separately. On the billing date, the bills of the daughter card are also included in the bills of the main card, which requires the main card to return the bills of the main card and the daughter card at the same time when repaying.

The secondary card supports separate repayment, and the secondary card holder can deposit the amount to be repaid into the secondary card for repayment, so the primary card holder has no great repayment pressure.