Deciphering hotel room designation.

Room pricing is almost always related to the number of people. In the US we buy the room regardless (to a limit) of the number of people.

A double will be a double bed for two people. (Could also be two twins for two). A single will mean a twin bed for one person. A double for single use is a double bed for one person (more expensive than a single but less expensive than a double for two because it is per head pricing).

No, you cannot book a double for single and put two in. In Italy, they will copy the passports of everyone in the room so you cannot sneak someone extra in.

"Double for single use".
Room that could accommodate two persons (adults) but lodging only one person.

"Double for double use"
Same or similar room as above but lodging two persons.

You may find that there same double room may cost more for two people than for one. Two in one room with a double bed will still be cheaper on a per person basis than two single rooms.

Two people who share a room must book a double. Not a "double for single use" because that means only one person. As noted above, hotels in Europe tend to charge by the number of people, not just for the room like in the US.

In Norway and Finland a "single" room is intended for one person with fewer sq meters of floor area than a double and what Americans term a "twin" bed even though it is intended for one person.

Unanimous agreement. Double for single use is a room that can accommodate two people but is priced for one occupant. A second, "surprise"occupant will require payment of the double-for-double rate.

As a frequent single traveler, I'm glad that there's often some discount when I book a double room for single use (likely using fewer showers, breakfasts, Wi-Fi bandwidth, etc.) Unfortunately, it's not a 50% discount. Most hotels don't have a lot of true single rooms, since those are less flexible and less requested.

A second, "surprise"occupant will require payment of the double-for-double rate.

agreed.

Also, if you are particularly unlucky, you may very well get the "bum's rush", losing your deposit and your room at one go.

The fire authorities, the licensing councils, and the owners take a very very dim view of people staying over the stated limits.

Agreed with previous posters. Two people in one room must pay the double for double price. Most hotels serve breakfast included in the price and this is the main reason for the extra expense.

Second set of towels and toiletries can cost as much as breakfast, although I have seen some remarkable buffet plates.