Rule 355.11.b.1
Any legal subset must include the maximum number of legal targets possible for that subset. Example: A player plays Bellow’s Breath, a spell that reads in part “Deal 1 to up to 3 units at the same location.” They choose 3 units at the same battlefield. In reaction to Bellow’s Breath, the controller of those units plays Flash, moving two of them to base. On resolution of Bellow’s Breath, Bellow’s Breath’s controller cannot choose one unit in base and not the other. They may choose either of the legal subsets; the one unit remaining at the battlefield, or the two units in base.