doc says:
Base.create
static Function Base.create ( name , main , extensions )

....
Prototype properties or methods can be added to the new class, using the px argument (similar to Y.extend).

Static properties or methods can be added to the new class, using the sx argument (similar to Y.extend).

Parameters:
name <Function> The name of the newly created class. Used to defined the NAME property for the new class.
main <Function> The base class which the new class should extend. This class needs to be Base or a class derived from base (e.g. Widget).
extensions <Function[]> The list of extensions which will be mixed into the built class.

actual signature is:

Base.create = function(name, base, extensions, px, sx)