Level 3
You have to keep in mind Stripe can be used for a number of different applications. Like say you had a site where you could purchase things, like Amazon. It would be good if that site had the ability to have multiple cards, so you could say use your debit card for small transactions and a credit card for large. Thus Stripe also allows that ability.
By default if you only have one card that card will get charged when you make a transaction.
I hope that somewhat answers your question.