Lynn, TAT=Turn Around Time (not a defined phrase as per the SDD-service delivary document, but we use it for TTO+TTR).
eg. in one of our deliveries, Time to own the tickets is 15 minutes and Time to resolve the tickets is 3hrs 30 mins. so TAT becomes 3 hrs 45 mins (for Priority 1 tickets).
correction TTR= Time to Resolve and not time to respond as i mentioned in the previous post.