Categories are a set of pre-defined data types we allow you to map your data into, these are provided as blueprints for processing data in our platform, and in the future we may allow you to create your own.
They are presented in a format which looks like this:
interaction.type : string().required()
You can see it defines a JSON field using dot notation, the same way you'd reference it in CSDL. The right hand side then contains a type, and any additional constraints, as chained function calls.