I don't know if this applies to all classic cars, but the way the Corvette people classify a "Survivor" is the following:
At lease 50% unrestored in 3 out of the 4 following categories:
1. Under Hood
2. Interior
3. Exterior
4. Chassis
A friend has two cars that were awarded the Survivor tag at Bloomington Gold....A '62 roadster and a '66 427 Coupe....Both were 100% in all 4 categories, and both are stunning cars.