Macro functions can be used in any expression and always return a value (of type string, number, date, or Boolean). To easily identify the variable type for the parameters, each parameter name is prefixed with a lower case letter indicating the expected variable type - s = string, n = number, d = date, b = Boolean. If you don't pass the correct type to the function your macro will abort with the "Invalid expression in macro" error. The item after the : (colon) indicates what type of variable value is returned from this function.
For a complete list of available functions click here