This morph developed out of the flame morph and was observed and described in specimens as early as 2000. It was not until subsequent refined generations that harlequin became more apparent as being part of the flame trait.
Harlequin crested geckos are like flames, but with more pattern. Harlequins will show pattern along the lower laterals and limbs. 
The pattern difference between flame and harley is that harley develops significantly more pattern in the limbs and belly. The most extreme examples reaching up and over the top of the dorsum. The pattern is mostly comprised of several overlapping oval-shaped markings or blob shapes around the limbs and laterals. 
Although flame is credited for the origin of harlequin, today it behaves like its own morph. flame and harley are the same main traits at work with other genes contributing to the expression. They work the same to produce the same phenotypes with varying amounts of patterning. The extreme examples are referred to as extreme harlequin, full coverage, and even XXX, making them a sort of complex.