A Fashion illustration is any illustration or rendering that conveys a garment or style... it can be very precise and realistic or very vague. The end use or purpose of the drawing should dictate the style used. For example, if the illustration is to help a buyer visualize the collection or for advertising a garment, the drawing should be accurate to proportion, color and fabric. If it's a illustration more for the mood or style point of view (for say a magazine) then the drawing can be less literal.
In school, students are taught to draw fashion figures at a scale of 8 or 9 heads. This means if your head is say 1 inch long then the total figure would be 8 or 9 inches depending on your preference. I personally usually fall around 9.