It's a slit surrounded by green fabric, and appears to open up to a giant second layer in the jacket, that runs along the entire jacket seemingly, but there's no second exit hole, nor any explanation on the website or anywhere that I can find, for what it's used for.
Due to there only appearing to be one hole
(unless I somehow missed it after 30 minutes of checking)
I am completely at a loss as to what it's for - originally I would've assumed it was for earbuds, but now I have no idea.

Any idea as to what it's for?