@config default_config my_function(x; a, b) = ...

This macro creates a function my_function(config, x) which will have access to the variables a and b. The values a and b will be those in config, if they are present, or else those in default_config.

@default default_values my_function(x; a, b) = ...

This macro assigns the values in default_values as the default values for the keyword arguments a and b.