I know, it’s getting confusing: there are behaviors, there are parameters, there are parameter behaviors—and now, there is a Behavior parameter! Well, the Behavior parameter is a parameter that is only located in the Sequence Control group of the Sequence behavior, nowhere else—does that help? Maybe not.
At any rate, if you set the Behavior parameter to Custom, a new Location parameter appears below it that allows you to keyframe exactly how the sequencing animation moves through the text object. If you want the animation to only affect the first three letters or to move halfway through the text and then move backward, the Custom Behavior parameter can do the job.