The opt-in type determines the steps taken before a contact is subscribed to your product/service.

Single opt-in is a one-step process where users sending an opt-in keyword to your number are immediately added to the contact list selected in opt-in and can start receiving messages.

Double opt-in is a two-step process where users sending an opt-in keyword to your number should confirm their subscription with a confirmation keyword (defined by you such as CONFIRM, YES, etc.). When a customer replies with an opt-in keyword, an SMS will be sent to confirm their opt-in. Upon receiving confirmation, the number is added to the contact list selected in opt-in.