A buyer's PayPal Express Checkout payment failed due to a card-issuing bank denial. For example, the transaction exceeded the buyer's card limit.
If you send your buyer to the PayPal page to select an alternate funding source or add a new funding source, this eliminates the need to perform another SetExpressCheckout call that reinitiates the entire transaction flow. It also allows the buyer to complete the transaction instead of getting routed in a checkout loop.
See the Developer Portal for a complete list of NVP/SOAP API error codes.