With bank uploads, you no longer need to double handle your payments out of Rex. Previously, you would have to create the transaction in Rex, create it in your bank account, then repeat for any other transactions you need to do.
Now you can do all of your transactions in Rex, then at the end of the day, create an ABA file, which will include all of the transactions and upload that to your bank.
All the information about what funds need to be paid where are held in the file - so you bank will complete all of the transfers on your behalf.