This texture is used to communicate with the vertex coordinate system of hair strands. This provides the ability to use a gradient color for hair strands (figure 1).
Figure 1: Using the W Coordinate node to apply a gradient along the length of hair stands.
Usage
The following illustrates the material setup assigned to the hair object after creating the hair (figure 2). Once the gradient node is created, connect the W Coordinate node to the Input of the Gradient node.
Figure 2: The typical Node Editor setup for using the W Coordinate node with a material used for hair strand shading.