[web2py] What is polymodel?

[Jose]

in the log (of mercury) see:

changeset:   844:c32ab678e14a
user:        mdipie...@massimo-di-pierros-macbook.local
date:        Sat Sep 04 09:31:17 2010 -0500
summary:     polymodel support

What is polymodel?
Do you use?
How is it used?

Regards
Jose