It's the housing-suitability statement used in family reunification cases for minors. It declares that the home meets minimum standards for the number of family members who will live there.
Without the right S1 (or local equivalent), the family nulla osta application can't be processed. The comune's technical office typically issues or attaches it.
Some comuni issue an idoneità alloggiativa certificate instead. Both serve the same purpose. Ask your comune which form they use locally.
Official source
Ministero dell'Interno
Thanks, we'll take a look.
If you left an email, we'll reply when there's something to say.