Keep in mind that these TP Threshold alerts are for what TP thinks is your ANaerobic threshold, NOT AeT. That’s a big difference. The TP does this is that it sees how fast you’re running or how high a heart rate you are maintaining for an extended period. There, and a good definition of AnT is a pace or HR you can sustain for a long time. How long? 30-60 minutes depending on your level of fitness. So if TP sees that you sustained an average HR of 168 for 45 minutes and you previous highest HR sustained for that long was 162, TP will tell you you have raised your AnT.
Do not use HR zone calculators they do not work. Use your AeT as the top of Z2 and your AnT for the top of Z3. We have several articles on this site explaining how to find AeT and AnT.