ReAction features a flexible state editor to configure dynamic behavior of the bar.
Create a series of states, which will be evaluated in top-to-bottom order each time a corresponding game event occurs. The first one whose condition is true will apply its selected properties.
For example, a bar configured with 3 pages of actions could have page 1 shown in Battle Stance, page 2 shown in Berserker Stance, and page 3 shown in Defensive Stance. Or you could move the position or even show or hide it completely if you're a Warlock in Demon Form, or a Hunter with a pet out, or your target is hostile, etc.
Conditions are derived from WoW's macro conditional syntax, presented as a series of checkboxes. You can even specify a conditional directly in macro syntax if desired.