Users subscribe to issues that are of particular interest toy them. The general rule is that if a user subscribes to an issue they will receive an email every time that issue is update (although it is possible to disable update emails on a subscription - see below). Users can also use "subscriptions" as a condition to filter the issue list.
NOTE: The ability to add and view subscriptions on an issue is controlled at the group item security level therefore you may not have access to this feature, you may only have view access, you may only be able to subscribe yourself to issues or you may be able to subscribe other users to issues that are assigned to you.
On the right side of the Issue Details page, below the Attachments section is a list of users currently subscribed to this issue.
Users subscribe to issues my clicking on the Subscribe link on the top of an Issue Details page. If the user is already subscribed to the issue then link switches to Unsubscribe.
Depending on the site option settings for Issue Subscriptions and your group item security, there may be a New ... link beside the Subscribers allowing you to Subscribe other users from a pop up list of users that are members of the project the issue is part of. There will also be a check box to indicate whether this subscriber receives emails when the issue is updated.
Clicking on a user id in the subscription list will present the option to enable or disable email notifications for that user.
Users can be removed from the Subscriber list by checking the box beside the user id. Selected users will be deleted when the issue is saved.
