Different formulations contain different amounts of elemental calcium. More tablets are required to achieve the same dose. It has the highest percentage of elemental calcium, therefore, fewer doses are needed. Many generic products are available. Calcium carbonate must be soluble to be absorbed by the body.
Solubility is increased by acidity in the stomach.
Solubility and absorption are enhanced when taken with meals. It is more expensive that calcium carbonate, but does not need to be taken with meals.