Nikon's CLS is an optical communication system that uses infrared light (IR).
Range is limited by the low power of the built-in flash unit to some 10's of feet but can be extended somewhat by using a SU-800 instead.
Line-of-sight is usually required because of the high frequency of the IR signal. (The lower the frequency, the more a signal can 'bend' around objects)
More than 1/2 the light energy from the Sun is IR light so CLS outside can be overwhelmed making range even more limited and in direct sunlight may not work at all.
Radio triggers deliver a much stronger signal at a much lower frequency so their signal can penetrate walls, go around corners, and provide range of a couple of hundred feet
The IR light from the Sun has no effect on the radio signal so range and performance is not diminished by direct sunlight.
Radio triggers are 3rd party devices and for TTL use the communications protocols have to be reverse engineered.