A Time Trial is a competition between you and the clock (and everyone else and the clock!) You are on the track alone and must try to record the fastest sequence of laps without spinning, going off the track, or having contact with a barrier. Any such incident will invalidate your current lap sequence, and you will need to complete another full sequence in order to better your time.
Your Time Trial Best Time is your average lap time for your best sequence of laps in the session. This can range from 2 to 10 laps depending on track length. For example, WeatherTech Raceway at Laguna Seca requires four consecutive incident-free laps. After you have registered and joined a Time Trial Session, you can find this value in the Simulator on the “Info” panel.
Your Time Trial Best Time is maintained per Car Class, per Race Week, separately in each iRacing Series. If when you run a Time Trial you set a personal-best Time Trial Best Time for that Series, Race Week, and/or Car Class, that Time Trial Best Time is then compared to your previous Time Trial Best Time for that configuration. If the new session’s Time Trial Best Time is better, that will trigger an update to your Time Trial Rating (ttRating).
If a ttRating update is warranted, the ratio of your new Time Trial Best time to the World Record Time Trial Best Time for that Car Class/Track Configuration is computed. This "normalizes" different Car Class/Track Configurations to each other.
Your ttRating is seperate for each license class and is cacluated from your 8 most recent Time Trial results in that license class, regardless of the Car Class used.
What if you haven't time trialed at 8 tracks? What happens here, is that you're given the service-wide average rating for the remaining tracks. So the table shows Driver A having time trialed at 4 tracks, leaving them with 4 tracks at which they are getting the average rating.
Once you have Time Trialed at 8 tracks, your ttRating is more correct, because now you have 8 real ratings for 8 tracks, no service average ratings are included. Once you complete a Time Trial on a 9th track, only the 8 most recent are used, so in this case, your first track would be dropped from the average.
Having the best Time Trial Best Time or being at the top percentile of the standings for a given week or track doesn't necessarily equate to a positive gain in ttRating points.
As ttRating is a measure of competitive performance skill for time tirals, it is only adjusted up or down whenever a Time Trial event is completed. Becaues of this, setting a new World Record Time Trial Best Time could result in your ttRating lowering if the World Record Time Trial Best Time for some of the 8 previous Car/Track Configurations you ran may have been improved on since your ttRating was last recalculated.
Your iRating and ttRating will only appear on your competition license once you are awarded your Class "D" license.
You can find more information in the Sporting Code Section 4.4. Time Trial Rating