Subscribe to Campaign allows the subscriber to receive notifications for all new ideas and comments for that specific campaign.
Steps to subscribe/ unsubscribe campaign.
 1) Click on the desired campaign. You will see Subscribe to the campaign link under the About Tab just below the Campaign Question / Challenge Statement. Click on this link to enable idea/comment notification for a specific campaign.

Another way to subscribe to the campaign is by clicking on the Star icon beside the campaign name which is visible when you are scrolling down a campaign page. Blue star indicates the campaign is subscribed.

2) Subscribe campaign is like a toggle switch. Once the user subscribes to a campaign, they would see the Unsubscribe from the campaign option. Click on unsubscribe link or click on the Blue star (unsubscribed will make it White) to stop idea/comment notification for a particular campaign.

Help article on campaign settings:

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