These compare the volume over the last minute to the volume expected in a 1 minute period.  The result is always a percent.  If you set the minimum volume to 400 then you will only see stocks which have been trading at least 4 times as much as normal in the last minute.  Show me.  Set the maximum volume to 90 to see stocks that have been trading less than normal over the last minute.  Show me.

These filters work before and after regular market hours.  However, you should set them to lower values as stocks typically trade significantly less during these hours.