Nice pickup. That means that a Windows 10 Mobile device can use expanded storage, such as a powered USB drive. (USB Mass Storage (Bulk & UASP))
That has only been possible with Android using an OTG (On The Go) connector I believe, which applied some limitations. In an application I wanted to use, you couldn't power the Android phone while connecting it to an external drive, and you couldn't connect it to a USB hub to allow a USB drive plus a USB SDXC Card Reader to be connected, for backup purposes.
Of course it just hasn't been possible at all with iOS, as far as I can remember. (No doubt someone will correct me if I am wrong.
)
I want a decent Windows 10 Mobile phone. . . Still waiting. Getting closer.