The main card and the supplementary card share the same amount. For example, the credit line of the main card is 10000 yuan, and the credit line of the auxiliary card is also 10000 yuan. If the cardholder of the primary card has used the credit line of 5,000 yuan, the secondary card can only use the remaining credit line of 5,000 yuan at most. In addition, the main cardholder can specify the credit limit for the supplementary cardholder. For example, you can apply for a supplementary card with a credit limit of 10% for your child.