Knob behind arm rest - A321EOW

I fly a lot but cannot recall ever seeing this little selector knob. As you can see, it's on a panel attached to the rear of the armrest in front of me. I looked forward to the row ahead, and their jnob aligns with the black marks. Mine doesn't but I presume that's inconsequential.

What does this knob do? For clarity, it DOES NOT access life vests despite the signage. The life vests are accessed from the front center of the console via a large red nylon pull strap.