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Q: How can I tell what a decline code means?
A: Her's the Network Merchants Gateway (N.M.I) Response Code Lookup Table:
Q: In the Networks Merchants configuration section of the transaction gateways configuration page, what is the setting for that is titled "Use credit instead of refund"?
A: Typically merchant accounts want you to only send funds back to a card that was originally charged. If they allow you to use credit then you can send funds to an arbitrary card. That is a HUGE potential for fraudulent abuse. That being said, when you migrate from one merchant account to another, there can be a 30-60 day window at the start where you might need to have that enabled so you can refund arbitrary cards.
Refer to this list when reviewing the transaction history of an order.