This is an assurance that the product meets standards for purity and quality. TUMS is a good, cost effective source of calcium which meet the USP standard. Vitamin D helps your body absorb and utilize calcium, but it does not have to be taken with your calcium supplement. Unlike calcium, vitamin D can be stored by the body for extended periods of time. Your body can produce Vitamin D in your skin after exposure to the sunlight.
Vitamin D is also available through multiple food sources.
It is unlikely that you will get too much calcium. Typically, the excess which the body cannot use is excreted in the urine and stool. Most people absorb calcium with no problems.