Any labeling? Looks like a proprietary service port or LED connector. Other brands are doing the same and adding LED connectors on the boards, does asrock have led kits with that connector?
Only other thought might be bluetooth or wifi connector (no idea the function of the port though)
I don't see any relevant labels in close proximity. There is one label under ATX conector which says UART1/PWM CFG1. Is it 8 pin COM port then?
LED connectors are further to the right, next to CPU Fan connectors and another one right above PSU connector.