Noise works on how many pixels change in two subsequent flm generations of the scene. As the scene converges, less samples change up to a point where no samples will actually be different between two flm generating cycles. This lets you stop the scene when the changes would be virtually undetectable to the human eye.

Progress is just using the Samples per pixel criteria.

Personally I just use time, whether a scene is ready is subjective and I like to alternate scenes on a time basis.