Email notification means that when someone is assigned an item they receive an email notifying them of this fact. Email Notification on BrightWork lists is handled via the BrightWork settings page. When enabled it sends an email to the new Assignee of a list item.
The Project Tasks list in the Project Lite, Project Standard, and Project Structured templates is not a BrightWork list. It is the native SharePoint Tasks list.
SharePoint 2013 does not currently support assignee email notification in this list.
To enable email notification:
If a form does not have an assignee column (for example, the BrightWork Custom List) then add a user column called AssignedTo.
The name of the Assignee column can be updated after being added; however, for it to appear on the BrightWork Settings page, the AssignedTo title must be used initially.