Your Transaction Sources have a different function from your funding source. While your Transaction Sources are also external bank accounts that you connect to Peak though Plaid, they're only function is to track your daily spending in order to attain Round-ups (and to power Insights and Actions with Peak Premium).
As opposed to a Funding Source, Transaction Sources are completely optional within Peak, and we will never move money in or out of your Transaction Source. A great choice for a Transaction Source would be your credit card or any other account you commonly use for every-day purchases. Also, if you so choose, it is also possible to pick the same bank account, a checking account, to use as both a Funding Source and a Transaction Source.