When an email is added to multiple work items, one of these work items becomes the 'primary work item'. This primary work item is:
The work item that is listed under the email subject.
Where the email will be listed under if your are sorting your To-do list by Work.
The primary work item is automatically selected, based on the following criteria (in order):
Based on privacy settings of the work items (Public, Private, Hidden). eg. if only one work item is Public, and the rest are Private or Hidden, the Public work item will be the the primary work item for that email.
If there are more than one work items attached to that email, with the same privacy setting, then the first work item that the email was attached to will become the primary work item.
Once an email is attached to a work item, the email's Contact will be updated to become the client of that work item.
If you would like to change the primary work item for an email, simply remove the current work items from the email, then re-add them—with the primary work item chosen first.