This is the time between when a ticket is ASSIGNED to ANY AGENT TO when the FIRST public reply is SENT.
#1 This is when the ticket was created:
#2 This is when the first reply was sent:
#3 The FIRST RESPONSE TIME AFER ASSIGNED for this ticket is the time difference between #1 and #2.
Another way to understand this metric: The ticket MUST be assigned to someone in order to calculate this metric.