I read about HKWorkoutSession and they said that if I'm using HKWorkoutSession without using it for recording a workout app, it will be rejected on the App Store.
So what else is there that can keep running the app in background activity? Examples like Music App, Workout App, and Nike Running Club, if start they will have the app logo on the above screen like notification badges.
Also, if the user press the digital crown or open another app, it will automatically reopen after a few minutes of inactivity while the session is still running.