WordPress requires you to define a sanitize_callback for every option you create, which is a great for security purposes.
In Kirki depending on the control-type you’re using we’re automatically setting a sanitize_callback for all your fields so you don’t have to.
However, in case you want to override the default sanitization you can enter a callable function or method here. If you use a function then you must set this value to a string (the function name) and if you use a method then you must set this to an array. (example: [ $object, $method_name ]).