There are two reasons. The first one is: because your main and additional cards only have one limit, then if your main card has no limit, then naturally the additional card will not have the limit. Used.

The second situation: when you open an additional card, the main card sets a flow control for the additional card, that is, the additional card can only use this set amount in one billing cycle. This traffic, regardless of whether there is an amount available in the main card, cannot be consumed by the additional card. If you don't understand, you can ask. Anyway, I'm fine lately.