Calcium

Both are calcium salts: carbonate combines calcium with carbonic acid and citrate with citric acid. A salt weighs more than the calcium in it, which is why labels state the calcium itself and some titles say 'elemental calcium'. Compare the calcium per serving in the nutrition table, not the weight of the salt.

They are other calcium-containing ingredients that appear in titles. The ingredient list says what a product is made from, and the nutrition table shows whether calcium is declared and how much per serving, so compare the calcium line only between products that declare one.

Lactate is the salt of lactic acid and is a different substance from lactose, the milk sugar. If you avoid milk or lactose, check the ingredient list, where allergens such as milk are emphasised, and any statement on the label of the exact product.