That's pretty much right on the money. Black Oil Sunflower Seeds offer high levels of Acid Detergent Fiber as well as Neutral Detergent Fiber. They are also high in bypass protein.
Basically, limestone is the feed mills version of calcium carbonate. Ask specifically for calcium carbonate or a higher level of calcium and more than likely you will have feed grade limestone added to your feed.
As Roger said, it isn't as easily absorbed as other forms of chelated calcium supplements such as calcium aspartate, calcium lactate, calcium citrate, calcium gluconate, calcium orotate, calcium ascorbate and calcium malate, but is still a reasonable source of calcium for livestock.
Hope this helps answer your questions....
John
.