Mail can be tagged in Arctera eDiscovery to help with organization. In the event that a tag is no longer needed it can be removed entirely from Investigations or the Case.
Investigations
Log into Arctera eDiscovery and select the Investigations tab. Expand Tags and select the tag name from the expanded list. Select Delete Tag and click Yes to confirm the deletion.

Case
Log into Arctera eDiscovery and select the eDiscovery tab. Click on Cases to display the list of cases. Select the Case which will populate it under the Cases node. Expand Tags and select the tag name from the expanded list. Select Delete Tag and click Yes to confirm the deletion.

How to delete a tag in Arctera eDiscovery