A new subscriber asks to be subscribed to the mailing list, but unlike unconfirmed opt-in, a confirmation e-mail is sent to verify it was really them. Many believe the person must not be added to the mailing list unless an explicit step is taken, such as clicking a special web link or sending back a reply e-mail. This ensures that no person can subscribe someone else out of malice or error. Mail system administrators and non-spam mailing list operators refer to this as confirmed subscription or closed-loop opt-in.

Some marketers call closed loop opt-in "double opt-in."

The term double opt-in was coined by marketers in the late 90s to differentiate it from what they call single opt-in, where a new subscriber to an e-mail list gets a confirmation e-mail telling them they will begin to receive e-mails if they take no action. This is compared to double opt-in where the new subscriber must respond to the confirmation e-mail to be added to the list.

Some marketers contend that double opt-in is like asking for permission twice and that it constitutes unnecessary interference with someone who has already said they want to hear from the marketer.
Source: Wikipedia

Any form generated through CleverReach is double opt-in. You can change the content of your confirmation mails in your CleverReach backend.