Sorry fi this has already been explained in the forums, can’t find it:
Sorry we were using this but have been deprecitating it but might bring it back at some point. Right now it’s not used as far as I know. We removed it from the app but not from the website yet.
It’s basically your average heart rate at FTP (after 20 minutes or so). The last 5 mintues of a 20 minute threshold interval for example. SInce most people don’t do threshold for more than 20 minutes and maybe even rarely to 20-minutes it’s just an approximation.
Well, if it is not used, no reason for me to look further 
What about the weight sync?
Weight is used to calculate speed and distance.
Sorry if my question abouth weight in the initial message is not clear:
If I use the iOS app, my weight is updated on Apple Health, will the Trainer Day app automatically update my weight?
We don’t have a connection to Apple Health. If you use Apple Watch it does record your activity but we are really just doing Apple Watch so people can use HR.
It does record the activity far better than i.e. Zwift, that more often than not will report a single BPM value for all the activity.
Really impressed with the app. I was so frustrated with teh terrible UX of all other apps I tried, it is a real pleasure to get up on your bike and get everything working fast an easily. Coach Jack plan was easy to male and seems to really fit my needs. I Will miss the social aspects/gamification of the other apps, but I now I can see Twitch streams while I ride 
PS: but I still think it would be supercool if it could import my weight from Apple Health 