You may have noticed that while using your poker calculator it displays your hand odds while also using terminology like "hand rank", "group", or "group rank" all of which in some way or another refer to author David Sklansky's Group Hand ranking for hold'em poker. Originally described in the classic book, "Hold'em Poker for Advanced Players", Sklansky rated all the starting hands and put them in groups according to their similar win rate.
By clustering hands based on win rate and strength, it's easier to keep track of basic betting strategies associated with each individual hand. For example, in Sklansky Group 3 hands you will find 99, AQ, ATs, and JTs among others. The best cluster though is Sklansky's Group One which includes AA, AKs, KK, QQ, and JJ. They are going to show very high percentage win rates on your poker calculator as well as "raise, and re-raise" recommendations.
In adopting the Sklansky Group of Hands your poker calculator could in effect make you a "book player", because many, especially the mathematical poker calculators don't take other factors into account at the poker table. However, as a guideline, your poker calculator is going to have the exact odds, and correct mathematical indication served up for you, David Sklansky style.
Poker calculators have adopted this because, well they are just software designed by programmers, and not necessarily poker enthusiasts, but Sklansky is a Poker icon, educator, and author. I have had several poker calculators running at the same time for testing, and have found very similar results and percentage recommendations, because they generally use the same statistical backbone as Sklansky Group of Hands.
The difference between them lie in how their other features are factored in, such as how monitors your position, how many players in the pot, how many tight or aggressive players, stage of a tournament, and if a player's stake is up or down significantly.
Although published years ago, by using The Sklansky Group of Hands, poker software offers credibility to the ranking system, although it sure didn't need it. Professional players have known these rankings and what to do with them for years. Seasoned opponents will also know how to use them against you, if you are an obvious book player, so mixing it up is always a good idea.
Some other books published by David Sklansky include The Theory of Poker, Tournament Poker for Advanced Players, and Hold'em Poker.