Don't know about Ford, but with Chevy and GMC, a 'demo' to us usually means it was put into the 'courtesy transportation program' aka 'loaner' program, i.e. while your vehicle is in for service, you can drive one of our 2017 or 18 models. These vehicles are in that program for up to 3000 miles approx. After that, they are put on the lot as 'Demo' vehicles. The price tag is dropped roughly $2,000 by GM.
As for the warranty, it is 1,000 miles off the odometer. So, the warranty on Chevy and GMC is 3 years / 36,000 miles. If a demo has 2,900 miles on it, the warranty is 3 years / 34,100 (36,000 - 2,900 + 1,000) miles on it.
The 1,000 mile off odometer is because many vehicles are driven to the dealership from the factory instead of trucked to it. Also, dealerships trade vehicles with other dealerships all the time. GM makes the 1,000 allowance to cover that purpose.