A MappedinCategory is a collection of similar Locations.
A Category can have more properties such as 'name', or 'parents' (for hierarchical categories) as well. For example, you may have a Fashion category, which has a Men's Fashion and Women's Fashion category. A Men's clothing store would belong to the Men’s Fashion category, but not necessarily be present in the Fashion category's list of Locations.
The Mappedin 'things' object is where you would specify what properties you want to download for Categories. Only specify what you will actually use, to minmimze transfer time. Work with your Mappedin developer relations contact to set up any custom properties you need.
See below for an example a 'things' object with available Category properties specified:
categories: ['venue', 'name', 'language', 'externalId', 'parents'],
Locations in this Category.