GIB internally describes hands using the following features:
Cue bids, responses to Blackwood, and some other bids (e.g. help suit game tries) indicate or deny various combinations of honors in a suit. GIB explains this by stating the minimum honor holding it can have and/or the honors that it has denied. For instance, if it's cue bidding and bypasses 4♣ to bid 5♦, it will say ♦A, no ♣A.
When it shows some honor combination, it could be better. GIB uses an 8-4-2-1 system to calculate the value of honors: Ace = 8, King = 4, Queen = 2, Jack = 1. If it says that it has KQ of a suit, that's 4+2 = 6, but this is the minimum; it could have the Ace instead, since that's worth 8 by itself. This is somewhat like the old "quick tricks" evaluation of honor combinations (A and KQ When it shows some honor combination, it could be betterare both considered 1 quick trick).
Sometimes two bidding rules will result in the same bid. For instance, if your partner opens a weak 2, you would raise to game with a weak hand and 4-card support (raising the preempt based on the Law of Total Tricks), or with a strong hand and 2-card support (because you think you can make it). When combining rules like this, we use the minimum of each feature shown.
In the above example, the rule for the weak hand shows total points = 4+, length = 4+, while the rule for the strong hand shows total points = 16+, length = 2+. GIB is not able to describe a hand as showing "this or that", so it shows the least common denominator of each feature: 4+ total points, 2+ cards. This means that the hand has at least 4 points and at least 2-card support, although it can't have the minimum of both at the same time. Unfortunately, there's no way for you to tell that this is what it means.
When a player has made multiple bids in the auction, each of them contributes a specification. These are combined by taking the maximum of each feature. So if a player makes a bid that shows 5+ hearts, and later makes a bid that only shows 4, we will still display that he has shown 5+.
Sometimes you'll see a confusing description like 5+ ♦, 3-card ♦. How can it be both a 3-card suit and have 5+ cards, you ask. This is because these are two different features: 5+ comes from the Length feature, while 3-card comes from the Quality feature. Although these are related (a 5-card suit is always at least biddable), GIB doesn't take these relationships into account when describing the hand. If one bid only shows suit length, while another bid only shows suit quality, the final description will show each of these as they were shown by their respective bids. As mentioned above, we maximize each feature across multiple bids that show that feature, but we don't reconcile different features, even if they're related.
When humans play bridge, they don't just follow rote rules for bidding; they often use their judgement to find better bids, or fill in holes in their system. We would love it if we could program judgement into GIB, but that would be pretty advanced artificial intelligence. As with many game-playing computer programs (e.g. chess programs that routinely beat grandmasters), we substitute brute computational power for thinking. Many of GIB's rules allow it to perform simulations.
GIB starts by finding the matching bid in its bidding rules (we call this the "book bid"). If simulations are allowed, it then makes some adjustments to its hand (adding a card to each suit, adding/subtracting a few total points) and finds the book bids for those similar hands. Then it deals out cards to the other hands at the table consistent with the rest of the auction. For each hand and possible bid, it does 2 things: