Hair length plugin is used for one simple purpose- to parametrically or manually change the length of a dense hair model. Note that it is only possible to shorten the hair, not make it longer. That is because hair length uses only the provided info; it cannot extrapolate new information. You can control hair length either through a random factor, or through a monotone texture map. Texture map will the the UV coordinates or world coordinates of the hair (whichever ones are available).

Parameters:

Randomize - Determines how random the relative hair length should be. A value of 0 leaves hair as it is, a value of 1 completely randomizes hair length.