U/dev is a user-space implementation to manage the special device-nodes in the /dev directory on your root filesystem. Kernel device drivers register kobjects, while the informations are exported via a virtual filesystem to /sys a u/dev daemon receivs event notifications (via a netlink socket) and creates the device-nodes accordingly.

What is special?

With u/dev you will only see the device-node that are present on your system (as it also was the case with DevFS formerly used by T2) and you can configure persistent and human-readable names like /dev/ide/host0/../disc in contrast to /dev/hda or to avoid long names group all discs in /dev/discs. The enforcement of persistent names for example allows to make sure that one device is always accessed via an given name no matter in which order the devices are powered up or plugged into the computer.