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Glad you found the comments useful.

OP:: "... i don’t understand why you think it’s  an ordinary  luminosity mask. I mean, yes the result will be a normal  mask, but the data is extracted from an RAW image, and it’s dynamic  range that holds these extra information, to create the luminosity mask.  - Let me know your opinion on that...."

The reason is simple: You can get the same effect from any normal JPG, TIF, or other image, as well from an image that has already gone through a raw converter.  One doesn't need the raw image to work from.  It's a more general technique than you suggest by your title, so why emphasize this.  If you are working on an image with 16 bpc, the effects will be almost indistinguishable.  If you are only using 8 bpc and the hair and background are both very similar dark tones, the mask you get from the rendered version will be a bit more crude than what you would get using 16 bpc or the raw data, but you're still making nothing more than a luminosity mask tailored to separation of two objects (ie, hair and local background) in the image.

HTH,

Tom