Wondering what this port is for? The user guide to Targus DOCK110 states "USB 2.0 Micro B for optional powered mode (DC 5V @ 1.5A, sold separately)". It is not a data port for connecting the DOCK110 to a host. This port is for supplying additional power to the DOCK110 that in-turn can be supplied to the USB3.0 ports. The DOCK110 is powered from the USB port that it is plugged into. If that host (i.e. a phone) does not supply power, then a power supply will be needed and this is where that power supply plugs-in. (In the example, use the phone's power supply). Plugging in an external power supply can also enable supplying additional power to the DOCK110's USB ports that can be used by devices plugged-in on these ports. Most PC/MacBook/Linux USB ports supply enough power for the DOCK110 for dual display monitor, GbE, and simple USB devices like a mouse and keyboard, or even a flash disk. However, many phones USB OTG do not supply power and so an external power supply may be necessary.