Nokia 2626

If u are any familiar with d nokia 2626 phone, u'd be well aware that there 3 ports @ d bottom of d phone:
charger, eye-piece & another that looks like a usb port.
On d user manual, users are strongly advised not 2 make use of that particular port, why?
Is it a usb port? If not so, then whats it 4?
What are d implications of using d port?