Only superusers can work with notification variables.
In addition to predefined variables, you can create your own substitution variables, for fixed text only. On the Settings page, click Notifications in the Setup area and go to the Variables tab.
Click the + sign to create a new variable, fill in the fields and click Save.
You can only add fixed-text notification variables. They will be added to the bottom of the reference list of variables on the Notifications Setup tab.
See also: Customizing Notifications