W axis is the along length of dynamic hair. BB did a post with an explanation of it years ago, but I can not find it with the search here.

Dynamic hair uses 4 axes. W,X,Y, and Z.... X and Y map where the hair comes out of the scalp. Which is why you could use a texture in firefly to make strands differing colors based on where they were on the scalp. I don't recall what Z does, and W is the along length of the hair. Basically if you want to change something (color, highlight, etc) along the length, you do it on W.

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