I should be able to work with a subset of data without all the cumbersome steps of creating new lists or exporting. I was simply trying to see how many contacts on my OPENED list were members of a particular tag. I admite that I did not fully understand the MANAGE TAGS only meant that I could apply a tag and not view a tag while in the OPENED list. And thus, I inadvertently applied a tag to all the contacts in the OPENED list - not what I was trying to do.
1-there is no undo
2-to filter to get to a subset of data from the email metrics should not be as cumbersome as to have to export or create new lists