The common notification rule is a global email notification rule which you can create and choose to serve as a default one for all newly created projects.

The common notification rule is overridden by project-level notifications rules. See Project email notifications for details. Too, notifications are sent for activities of any status Except if the activity is in a project with an equal to completed status.
Enabling/Disabling Common Notification Rule
To edit/disable a default notification rule (enabled by default):
- Log in as Administrator (or a user who has the ‘Manage System Settings’ permission). If you are already logged in, skip this step.
- Click through User menu | Settings | Notifications on the header menu.
- Select the Common notification rule tab.
- To disable: Clear the 'Make Common Notification Rule available in all projects' checkbox, or
- To edit: Make changes as required. On the Notify creator option, see below.
- You can select one of the 3 preset event selections: Key events, Nothing, or All(default).
- Click 'Apply'.
Use the same process to enable the default notification rule.
Notify object creator
This option is designed to allow the person who is the creator of a task, issue or request (not project, message or attachment) to be notified when other users make changes to the objects the person has created.

If you want to be notified of changes you make to the objects you have created (task, issue or request), make sure you have checkmarked the "Notify person who made changes".
Messages and attachments added (created) by a user are not covered by this option. In other words, if you are the creator of a message or attachment, and this option is enabled, you will not be notified of the event of creating those.