Please refer below to the differences between Issuer activation and Opt-In method:-
Issuer Activation: - This is typically used when a bank already has enrolled cardholders and wants to upload just the passwords onto our database. If the cardholders are not enrolled, and "Cardholder Not Uploaded" option is used with "Issuer Activation", then there should be callouts configured to somehow connect to the bank and retrieve cardholder information.
Opt In: - This is almost always used for Activation During Shopping (ADS). With Opt In - if "Cardholder not uploaded" option is chosen, then usually a callout will get answers to cardholder data real time from an external system, such as banks' database. If "Cardholder data uploaded" option is chosen, then banks' will upload cardholder data to be used for authenticating and registering a cardholder