When you make a payment out of Verto you are able to include a reference on this payment. This is often used to identify a particular transaction or to help assign a payment to a specific invoice.

To do this start a transfer from your wallet by selecting "Transfer money from my wallet" --> "To my Account"/"To a beneficiary"
​
You will see a "Reference" option here - here you can include a reference that will be attached to the outgoing payment.