I think the x's are to show where the background coloring is to go. The lines are probably for some kind of shading. I'm not sure about that circle on her right breast.
If I were to guess, I would say the lines denote shadows, while the circle specifies a bright shading point.
I remember old books about how to draw manga where they used X's to mark all fill black spaces. Obviously that only applies to monochrome without shades.
