It can be used in the body too, actually. I don't think there are a lot of rules really, as to what to use where. It is more personal preference. Waterproof fabrics like PUL and heavy fleece on the outside of AIOs, pockets and covers, definitely. Wicking fabrics like microfleece and suedecloth belong on the inside, next to the baby. It just wouldn't make sense to use these layers anywhere else.
Most absorptive layers can be used anywhere, body or soaker, especially for fitteds. And in any combination you desire. So go ahead and use just fleece or interlock or flannel, or mix them up. My BKT diaper has one body layer and one soaker layer, combined with a layer of cotton knit in the body. With the matching lay-in two-layer BKT doubler, it is very absorbant.