One issue I have with Explore is the fact when you use the Attribute of Assignee the list includes not only your Agents but also all the end users in your system. I have confirmed this with ZD Support:
At this time, unfortunately the Assignee attribute will display the list of all users from your account and this isn't already filtered for agent only in your account and you will need to create a calculated attribute to fix this issue at this time.
I don't know how you can assign a ticket to an end user or even light agent. Why should they been in the list?
This choice list should be filtered without the need for a calculated attribute field.