Esse pino extra é usado para ID (para OTG USB). Está normalmente em curto com GND via resistor ou não conectado.
OTG mini plugs The USB On-The-Go standard introduces a new plug receptacle called mini-AB. It can accept either a mini-A plug or a mini-B plug. Mini-A Adapters allow connection to standard-A USB cables, coming from peripherals. The standard OTG cable has a mini-A plug on one side and a mini-B plug on the other (it cannot have two plugs of the same type). The device that has a mini-A plugged in becomes an OTG A-device, and the one that has mini-B plugged becomes a B-device (see above). The type of the plug inserted is detected by the state of the pin ID (the mini-A plug has the ID pin grounded while the ID in the mini-B plug is floating). (There are also pure Mini-A plugs, used where a compact host port is needed but OTG is not supported.)