The "xxlimited" module (Modules/xxlimited.c) was added as part of PEP 
384 (Defining a Stable ABI), and is undocumented. As far as I can tell, 
it was added partly to test the stable ABI, and partly as an example of 
how to write a module (like "xx" from xxmodule.c).
In the last few years the module has not seen much maintenance, and I 
believe it's no longer a good example to follow: it works, but there are 
now better ways to do things.

I would like to take over maintenance of the module and make it into an 
example of how to write a low-level C extension with isolated module 
state, as described in PEP 630 (Isolating Extension Modules) -- an 
informational PEP that I plan to convert to a HOWTO doc when everything 
is ready.

The old module will be kept around to test the 3.5 stable ABI.

Past discussion: https://mail.python.org/archives/list/python-dev@python.org/thread/FO3YPG3YLG2XF5FKHICJHNINSPY4OHEL/#YITRQXGUOIEHK22QP5K4C5E45QA356U3