generally, the odd number sizing (at least in the States) will put one in the Juniors department while the even number sizes could put one in Misses or Petites or potentially even in what they refer to in some stores as the women's section (often size 16 and larger).
For many years, I wore a Juniors 3 or 5 in jeans...but then in my mid-30's, I started putting on weight because I was spending more time in an office environment and had given up a sport I had been very active in for many years...that gain would ultimately see me balloon to a Misses 10 then a 12. When I could not get a jacket on a size 12 suit to button, I took steps to lose some weight and am now back down to an 8 for shorts and slacks within just a few months of having begun that effort, with a potential of being able to get back into some of the juniors size stuff still in the closet...