PayPal Express Checkout works by allowing buyers to utilize their PayPal digital wallet in order to make purchases. This digital wallet may consist of bank account(s), debit card(s), and/or credit card(s).
During checkout, the buyer chooses which funding method they would like to use for this particular transaction. If the funding source chosen was not available for any reason, though, this results in a failure. For example, if a buyer chose to use a debit card, but that particular card’s daily maximum with the bank was already reached, the order would simply fail.
In the past, all developers could do was send the user back to the website and display some sort of error or instruct the user to try again with a different funding source. This caused confusion and often resulted in dropped orders, which isn’t good for anybody.
PayPal responded by introducing a new error code (10486) and functionality specific to handling this sort of scenario. Now, instead of redirecting users back to your site and displaying a message there to retry the order, you can redirect the user back to the PayPal Express Checkout page so the user can quickly make the change and continue from there. This results in fewer drops and helps maintain or increase conversion rates on your site.
PayPal provides more details about this on the developer site. You definitely want to make sure you’re familiar with this and you’re handling it correctly on your websites and applications.