Q. What is the purpose of ranking under category manager?

Category ranking is a feature when you are using the Power Add-on  (for the self managed WPSLP plugin) or the Professional or higher level  subscription for MYSLP SaaS  with the category manager. The rank is used to sort the categories which will affect the map marker that is used when a location has been assigned more than one category. Following the page ranking paradigm set by WordPress, the rank field is a simple numeric weighting for the categories. This extended data attribute for categories makes it easy to re-arrange the “weight” of categories for map marker selection without mangling category slugs. The lower the number, the higher the precedence for the map marker.

The option to set a category rank can be found under the Store Locator Plus Categories tab.