Round-Ups are transferred from the checking account that you link to Acorns. If you link multiple credit or debit cards, then all of your Round-Ups will still come from only the linked checking account. This checking account is also the account that funds are sent back to when a withdrawal is requested. For example, if you spend $3.50 using your credit card then that $.50 will be transferred from your checking account into your Acorns account when the Round-Ups add up to at least $5. You can see the account you have linked by going to Settings and selecting the Funding Source menu item.