You may be aware there are 2 significant (and significantly different) metabolic break points called “thresholds”. One is the aerobic threshold and one the anaerobic threshold. TP is referring to the Anaerobic one in their threshold notifications.
The significance in their difference is important for your training and they should not be confused. If you are unclear about this subject we have copious amounts of information on the site and in our book about this distinction.
Scott