It's the address where you have your main centre of affairs, often where you receive official mail. It can be the same as your residenza, but it doesn't have to be.
Banks, tax authorities, and lawyers often ask for both residenza and domicilio. Splitting them is common for people who travel for work or who use a different address for business correspondence.
"Frédéric, who lived in a holiday house in Stintino but had his business operations in Sassari, set his domicilio at the business address. Mail and tax notifications all arrived there, even though his residenza stayed at the holiday house."
Domicilio is about correspondence and centre of affairs, not about where you actually sleep. Residenza is about where you actually live and is registered in the anagrafe.
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.