This is calcium carbonate and is not absorbed as well as calcium citrate (elemental). See below.
For more:
https://www.dsfacts.com/calcium.php
From DS facts:
Understanding Vitamin and Mineral Supplement Labels
Vitamin and mineral supplement basics continues with a lesson on how to read vitamin labels. A patient has to read the fine print; the labels on these bottles can be very deceiving. Pay particular attention to the dosages. What it says in big print on the label may not be per tablet and may need anywhere from 2-6 tablets to obtain that amount.
It is very important to understand the mineral potency of the supplements. We will use calcium as an example. Elemental calcium is the amount of calcium that is actually in the supplement from the source. (The same is true for all other minerals.)
Example A: Elemental Calcium: 315mg
This means there is 315mg of calcium per tablet.
Example B: Calcium (Citrate): 500mg
When the source of calcium appears in parentheses it means the amount of calcium listed is elemental calcium, so in this example a recipient is receiving 500mg of calcium.
Example C: Calcium Citrate: 500mg
When the source does not appear in parentheses it means it, the 500mg, is not a elemental calcium. Since calcium citrate is 21% calcium the recipient would actually only be receiving 105mg of calcium in this example.