Not sure if this is a mistake in SmugMug's API or not, but smugmug.subcategories.rename documentation has Extras as a possible parameter. The description is the same as it is on all the other calls:
A comma separated string of additional attributes to return in the response.
The reason I ask is this call essentially returns a status and not any other details according to the documentation:
Returns an empty successful response, if it completes without error.
So is this a mistake? If this is not a mistake what does this parameter do?