Here's an example. Let's say you want miki to wear Diva hair.
Load Miki. Then open the folder that has Diva hair in it. This would be located in either your HAIR library or Character library. Load that hair into your scene. 
Then conform that hair to Miki. It will look really weird at this point. 
Now open up your Pose library and open the folder with the Diva hair fit for miki.
Take a quick look to be sure Diva hair is  still selected in your scene. Then click on the hair fit in your pose file. 

The fit files in the pose library tell the hair WHAT to do, they don't make the hair.