Todor Nikolov writes:
Extensions (or Featuresets) are a somewhat hidden feature of Rigify. They provide additional or alternative rigs that you may find useful. In the video below, I'll show you where to find such extensions, how to download and install them.
There is a quick overview of the rigs but showing how to use them is beyond the scope of the tutorial, sorry :) Discussing each new rig would require a separate tutorial and in future videos, I may go into more detail about using the new rigs.
Links to all Featuresets mentioned in the video: