Windows 10 Mobile will bring support for USB peripherals like keyboards, mice and memory sticks to Windows Phone, but a page directed at hardware OEMs reveals it will support a lot more, and also what support it will not bring.
In the post, Microsoft reveals support for USB Audio in and out, Bluetooth dongles and USB Hubs.
Windows 10 Mobile will however not initially support USB Printing, USB webcams and scanning, and more (see the table below) as Microsoft wanted to keep the storage footprint low for all the drivers, and did not anticipate the use of these peripherals on mobile.
|USB Host class drivers||Windows 10 Technical Preview for phones||Windows 10 Technical Preview for desktop editions (Home, Pro, and Enterprise)|
|USB Hubs (USBHUB)||Yes||Yes (Since Windows 2000)|
|HID – Keyboard/Mice (HidClass, KBDCLass, MouClass, KBDHid, MouHid)||Yes||Yes (Since Windows 2000)|
|USB Mass Storage (Bulk & UASP)||Yes||Yes (Since Windows 2000)|
|Generic USB Host Driver (WinUSB)||Yes||Yes (Since Windows Vista)|
|USB Audio in / out (USBAUDIO)||Yes||Yes (Since Windows XP)|
|Serial Devices (USBSER)||Yes||Yes (Since Windows 10 Technical Preview)|
|Bluetooth (BTHUSB)||Yes||Yes (Since Windows XP)|
|Print (usbprint)||No||Yes (Since Windows XP)|
|Scanning (USBSCAN)||No||Yes (Since Windows 2000)|
|WebCam (USBVIDEO)||No||Yes (Since Windows Vista)|
|Media Transfer Protocol (MTP Initiator)||No||Yes (Since Windows Vista)|
|Remote NDIS (RNDIS)||No||Yes (Since Windows XP)|
|IP over USB (IPoverUSB)||No||Yes (New for Windows 10 Technical Preview)|
Microsoft did however note that OEMs will be able to add their own drivers to phones in ROMs, meaning they could add support for any custom hardware on an as needed basis.
Read more about the implementation at Microsoft here.