When calling subscribe on a list where requires_confirmation is enabled, a subscription will be created with a status set to unconfirmed. An email will be sent to the email address you're subscribing. The email contains a link that, when clicked, will confirm the subscription. When a subscription is confirmed, its status will be set to subscribed.
When sending a campaign to an email list, only subscribers that have a subscription with status subscribed will receive the campaign.
To immediately subscribe someone, and skip sending a confirmation email, you can call subscribeNow on a list.
#Customizing the subscription confirmation response
When a person clicks the email confirmation link, a simple message explaining the result of confirmation is displayed. You can customize that response by publishing the views.