Not Available in Email Report

"Not Available" looks like a custom value that is being set by your implementation.....

The standard value for an empty eVar in Workspaces is "unspecified" and there is a standard filter to include or not include that default....  but this is text that has been specifically set in your dimension (either as a default value, or this is literally the extracted value)... I can guess that this is the equivalent of no value... 

However, to confirm this, you should consult with your team, particularly the people responsible for determining how things should track and are creating your requirement documents (assuming you have this), or possibly with your implementation team.... Understanding where and how this is being set will also provide details about what it means.