The Transaction operation should be performed if you are using a PreAuthorization flow. In that flow, the initial Checkout call to Signifyd occurs before you call your Payment Gateway for Payment Authorization.
After you call your Gateway, perform this Transaction call and pass the complete transaction information. The transactions array passed in this call will completely replace the pre-existing transactions array on the Order. Therefore, you are required to send the data elements sent in the Checkout call, as well as the information gathered after the authorization. This completes the journey of the transaction until another event occurs (such as a void or refund). You will need the associated checkoutId in order to make this Transaction call.