The Surefire U2 Ultra is a Digitally-Controlled 5 watt LED flashlight with 6 brightness levels.
Turning the collar changes the brightness from 2 lumens at the lowest level to 100 lumens at the brightest.
It uses digital regulation circuitry on all brightness levels to achieve a constant level of brightness for the usable life
of it's 2 CR123A size batteries. The U2's runtime is variable depending on the brightness level.
The U2 is similar to the L5 in that they both have similar size reflectors and beam profiles and they both use 2 SF123's.
The L5 has a combat grip body style where the U2 uses a more standard knurled grip body.
Here is the U2 taken apart. All of the circuitry is inside the head (bezel).
The U2 has a new style bezel down clip that also works well as an anti-roll device.
Beamshot comparisons:
The beam of the U2 (on the left) is tighter overall and brighter (obviously) than the Arc4+ on the right.
The U2 carries it's punch a lot farther than the lower powered Arc4.