umm i dont know what it is exactly called.. but it looklike a collar, not just a two pieces pad jointed on the back, u could see it on game designs, or julia voth cosplay costume... it makes the neck part looks better and look much like a vest not feel like Jill is just wearing tshirt and pad on her shoulder..
you could see it on the neck part:
even someone who customized it with other customizer such as psycho custom doing it right (sorry for using ur image, i know it belong to someone here
)
i dont know why they dont include it, it's not hard to make it, if they make the vest following the design while looking on several source images on STARS Jill they should notice it and know it.. i know we can always add it/custom it ourself after we buy it but it will be nice if for once we dont need to do it after buy the whole figures...