Acid-Free C is derived from calcium ascorbate. A natural form of vitamin C. Calcium ascorbate is not acidic, therefore, people can take larger amounts without gastric upsets or diarrhea.
Equally important is that calcium ascorbate replenishes the calcium that is excreted in the urine due to the chelating properties of vitamin C.
The more common vitamin C, ascorbic acid, presents no problem to most people at most dosages, but the greater tolerance of the digestive system to the neutral (non-acid) calcium ascorbate can be lifesaving for some. And it is neutral without having to react in the stomach with harsh gas-producing buffers used in "buffered" vitamin C formulas.