This utility node outputs the position along the strand of the hair. Some texture shader nodes will automatically use the hair position when applied to hair, but nodes that do not allow for a default uv coordinate cluster, or have an explicit uv coordinate input, you can use this node to drive the texture coordinate.
This outputs the position to the u component of the vector.
This is the default and outputs the position to the v component of the vector.
This outputs the position to the w component of the vector.