In Bridge Compass you have two different kinds of hands: a fixed hand and one made out of rules (patterns). The first is simple, you pick 13 cards. The second option has one or more patterns describing what a hand should look like. E.g., 15-17 HCP with a balanced distribution.
Defining a hand correctly is a difficult task. Suppose you play a natural system and want to define a 1♠ opening. Basically, you have 12-19 HCP with at least 5 spades. But what if you have an 11 count with a singleton? Or a ten count with a 6-5? To get a decent answer you don't have to be a 100% right but it should be reasonable. So, the 11 count with a singleton should be definitely in. The 6-5 with a ten count has a much lower frequency so it might not influence much and could be compensated by the 20 counts with a bare king that you open with 1♠ too.
Hands are defined on the pattern screen by clicking on the North, East, South or West button. But, on the main menu you have an option to switch the complete definitions between seats easily.
Note: when you save the hand definitions or switch them, the current set of deals and results will be deleted.