THR (Threshold Heart Rate, cTHR = cycling Threshold Heart Rate) represents the maximal sustainable heart rate for steady-state efforts lasting 30-75 minutes. Heart Rate to Effort (power) ratio is never linear, but under well rested, well hydrated/fueled, under normal temperatures (not super hot or super cold), it gives us a value that can be helpful in gauging training effort.
The cTHR value is used to set heart rate training zones. Your heart rate value represents the 3rd point in your normal training triangulation - Power - RPE - HR.
Monitoring your heart rate relative to your THR allows you to better understand how your training is impacting your performance (when compared to other metrics like RPE and Power)
The SYSTM app calculates cTHR in Full Frontal by taking 98% of your average heart rate during the 20-minute effort. In Half Monty, cTHR is calculated from peak heart rate values during the ramp using a special formula.
For more information about heart rate and heart rate training, be sure to check out the Wahoo Fitness blog: Everything you want to know about Heart Rate Training