In the rangemap command, you can use any categorization scheme that you like. You could call the categories A1, B2, C3 or just about anything else.
However, most people use one of these categories: low, guarded, elevated, high and severe - because the default Splunk CSS has already mapped these categories to the colors: green, blue, yellow, orange and red, respectively.
You can make your own categories by editing the application.css for a particular app. For example this defines a color for a Single Value Panel: