hotel jargon

I'm guessing the difference for one person using a room or two people.

It's very common for hotels in Italy to quote room rates per person. That may be why you're seeing these terms.

You cannot depend on a double being a double bed but it does mean two people. We have found double rooms with two single twins pushed side by side and sometimes two twins sheeted together to look like a double. Sometimes with a small double and sometimes a big double. But the phrase "double single use" does means one person in a room that normally has two.

If traveling alone, double single use can sometimes be a great deal. I've seen price differences as high as 30 euro on a double room for single use versus use by two. That's much more than the cost of the second breakfast, which is the most common price break offered to singles by a hotel that doesn't have singles or doesn't have one available. As an added bonus, double rooms are almost always much larger than single rooms.

Basically, Double single use is a method to get a little more for a room from a single that normally could be used for a double. If they have single rooms, you may note that the rate is less than Double single use, and the rate for a double more. Otherwise, it is not uncommon that if a solo traveller shows at a hotel, and no single is available, the hotel will often just put them in a double and charge for a single.