Somebody asked me about TT and what it was all about. Here's a clipping from the NASA website.

The National Auto Sport Association (NASA) is in its 3rd full season with its Time Trial Program. Last year, we had five NASA regions participate, with over 300 drivers nationwide. For 2006, we expect to have full participation from our eleven NASA regions, with over one hundred thirty-five events nationwide. We will be having a National Championship event at the Mid Ohio Sports Car Course in September, where we will crown our first National TT Champions. NASA TT utilizes a car classification system that is easy to understand, and gives HPDE-4 drivers a taste of competition by allowing them to compete for top lap times, in open passing run sessions, against similarly prepared cars for awards, prizes, and recognition.

In total, there are ten Time Trial classes, including one unlimited, one semi-unlimited, and eight limited classes. Each region has its own Regional Championship Series that gives us our regional champions, and qualifies drivers to become eligible to compete at the NASA National Championships at Mid-Ohio. Awards are usually provided to our top-level finishers at each of the regional events. To compete in NASA TT, a driver must obtain a NASA National TT License, which requires the driver to be an advanced level driver or racer. Driver and vehicle safety are among our top concerns, and we have a pro-active safety program. Our HPDE program, that accommodates drivers of all experience levels, is the best way for a driver to become qualified to compete in Time Trial. The Time Trial program completes the NASA competition pathway from NASA-X to wheel-to-wheel racing.

Hope this explains the difference between "Track Events", "HPDE", "Autocross" and "Time Trials".