The arguments that can be passed in the $args parameter are below. It is also possible to pass a WP_Error object in either the 'id' or 'data' argument. The parameter isn't actually optional, content should be given in order to send the correct response.

'what' argument is a string that is the XMLRPC response type. 'action' argument is a boolean or string that acts like a nonce. 'id' argument can be WP_Error or an integer. 'old_id' argument is false by default or an integer of the previous ID. 'position' argument is an integer or a string with -1 = top, 1 = bottom, html ID = after, -html ID = before. 'data' argument is a string with the content or message. 'supplemental' argument is an array of strings that will be children of the supplemental element.