Points in the Red
Points in the Red is the portion of your Suffer Score that you earned in your Zone 4 and Zone 5 heart rate zones. Points in the Red shows you the intensity of your workout. If your Points in the Red are 90% of your Suffer Score or higher, your activity is considered a Sufferfest.
https://support.strava.com/hc/en-us/articles/216917077-Suffer-Score-Premium-?mobile_site=true