Ah, my point was that you can also have a list of arrays, or a list of an array and a string, ...
So that's why I said it should be `column name, Grouper, array, or list of them` (with the 'array' before 'list'). 
I am also not fully sure what you mean with `it is being used as a mapping from data to new index labels`. It is just used in the same way as eg the column values.