Whitelisting is the practice of explicitly allowing some identified entities access to a particular privilege, service, mobility, access or recognition. It is the opposite of blacklisting. The term, as well as its antonym, are increasingly being deprecated in favor of language that is considered more inclusive.
Blacklists and whitelists are among the top methods used by companies in fraud management. Rather than blacklist and block transactions like these (that your gateway perceives are fraudulent), it might make sense to allow the transactions and flag it for a review later (whitelisting and calling these transactions safe can be just as bad).
Customers on the blacklist are generally classified as unsafe. In contrast, the whitelist contains all customers, which are called safe.