Resource Categories are overarching types of the resource you are loaning out. For example, if you loan/hire out instruments, these would be your instrument families such as; Strings, Woodwind, Brass or Percussion. They are the top-level definitions of your resources with two further categories (Resource Groups and Types) sitting under these definitions.
How to add Resource Categories:
Go to Settings 🡪 Resource Categories (under Resource Section) and add a new record:
Add the name of the Resource Category and the Resource Category Type, not sure what this is?
Then save the record where it will appear in the list on the left-hand side of the screen: