Please, what is this socket for?
That's for charging a 3800mAh battery using the side jack. To charge the BL-5L 3800mAh battery you must purchase a: - TYT TH-UVF9 battery charger for 110VAC or - TYT TH-UVF9D for 12VDC.
- These are fully regulated chargers
- Plugs directly into side of battery, not a charger base
- The AC wall wart has a charging indicator. Red while charging, Flashing Green Red when charged.
Check point no 6.7 on this page - http://www.miklor.com/uv5r/UV5R-FAQ.php#Batteries
It talks about an extended battery like you have shown here. Also says that it uses a side jack. The port you see here is probably for the side jack.