It's a watch pocket, originally for men who wore pocket watches and needed a protective place to store them.

这其实是一个表袋，最初是为有怀表的人设计的，怀表放在这里面刚刚好。

youdao

The micoach Elite System includes a small data cell that fits into a player's base layer in a protective pocket on the back between the shoulder blades.

ENGADGET: Adidas miCoach Elite System set to equip all MLS teams in 2013, creates world's first 'smart league'