Range is a group of hands played under specific circumstances (position, previous actions by opponents, perceived ranges).
Criteria for evaluation of hands to include in a range include:
Individual hands (included in out default ranges) are evaluated in Hand Quality articles.
Range balance is a special criterion which we only use in range construction. It basically addresses the need to have the ability to present a threat to our opponents on the majority of possible boards (there will always be a few where nobody realistically has anything better than a set). Otherwise we lose too many pots because alert opponents can attack us with impunity, especially when we are out of position (bet almost every check and raise almost every continuation bet).
Playing a balanced game by default is a reasonable strategy, but it is worth mentioning that it is really necessary to pay attention to range balance only against opponents who have access to data about our play (previous experience and, in case of online play, data mining / hand sharing). In other cases, it might be advisable to start with unbalanced ranges and then transfer to balanced ones after a while.
To illustrate the implications of range balance, I will use an example with 2 ranges of exactly the same size but different composition.
Range 1 (136 hand combinations, VPIP = 10.26%), balanced composition:
Range 2 (136 hand combinations, VPIP = 10.26%), Broadway heavy.
Range 1 has 65.04% equity vs. random hands.
Range 2 has 67.32% equity vs. random hands.
Range 1 has 48.38% equity against Range 2 (due to more speculative hands with lower cards), but it is by far the better choice due to better balance.
Here are how ranges 1 and 2 perform when a card flops:
Note: Second column shows that out of 136 hand combinations in Range A, 46 hit the Ace, which is 41.07% coverage (third column) when the Ace falls.
The table shows that Range 1 is top heavy (due to better equity of Broadway hands) but still has some coverage on medium boards and even on very low boards. This allows for more continuation betting (c-betting) and winning some big pots when opponents try to attack a c-bet or float the c-bet and take the pot on the turn.
On the other hand, Range 2 strongly hits high boards but is weak and capped on medium and low boards (“capped” here means that the best hand is usually an overpair or a good flush draw). It is not possible to threaten opponents with such a range since they can comfortably make correct decisions (it pays off for them to draw against a capped range).
Both ranges will look the same in the opponents’ HUD (in online poker, see article “Important Tools for a Serious Poker Player”, but after a while, they will be able to see the composition (from showdowns).
After opponents notice it, “Range 2“ will be easily exploited by aggression against it in position on low boards (where the range has zero coverage) or on medium boards (with very little coverage). When that starts happening, it is advisable to switch to any more balanced range (“Range 1” is one example).
Range balance is useful in another way, which might not be immediately apparent. Having a varied mix of possible hands in our range allows us to mix our actions more effectively. This means we can better protect our checking range with check-raises (to discourage / punish automatic betting against checks) and use continuation bets as semi-bluffs more effectively (since it is plausible we have a playable hand on any type of board).