A Transaction can be rejected due to many error code . One of the error codes is is 500 - Declined

DC 500 declines are rejections due to the transaction being flagged as high-risk transactions by our fraud monitoring tool, The main objective is to protect the merchants from fraudulent transactions and prevent monetary losses due to chargebacks/ cardholder disputes.

By default, such transactions will be automatically reversed back to the cardholder as per the issuing bank’s internal credit policy and applicable time frame

To handle DC500 declines, we can activate a feature called “hold on reject". 

 Whenever such transactions are flagged, the merchant will have the ability to capture/ void the transactions manually.

Disclaimer: Please note that if Hold-on-reject is enabled, the merchant accepts full liability for capturing the flagged transaction. 

Enabling the Hold-on-reject feature is subject to risk team review of the merchant account.