The main point is: it depends on the operation. NotImplemented is a way to signal that an operation is not implemented. It can be used for whatever you want to use it for. You can design to call an operation "foo", and, if NotImplemented is returned, call "bar" instead.

If you want to know how a specific operation performs its fallback, you have to look in the documentation of the specific operation.

As an example for a method where some other fallback is used, see

http://docs.python.org/py3k/library/abc.html#abc.ABCMeta.__subclasshook__