Flat Hands have a 4333 or 4432 shape. They tend to underperform in a variety of situations.

1,2 & 3 Suited Hands: A 1 suited Hand (1 suiter) is one with an obvious single dominant suit which it clearly wants to declare as trumps e.g. a 6322. A 2 suiter has two such suits e.g. 5532. A 3 suited is either a 4441 or a 544.

Total Tricks Principle (TTP)
states that when both side hold approximately equal points or when your side holds the minority of the points, it is safe to compete to the number of tricks your side holds in its longest suit. When vulnerable or with poor trumps or with a flat hand, it is prudent to reduce the number of tricks competed for by one. This principle underpins the modern aggressive styles of competitive and of pre-emptive bidding.

is described as being favorable when “they” are vulnerable and “we” are not; unfavorable when “we” are vulnerable and “they” are not and equal or neutral when both sides are vulnerable or both sides are not.