When multiple sets of scores are sent to a university, the university takes the highest of the individual section scores across multiple exams, and combines them to create an ultimate superscore.
For example, if an SAT student receives a Reading score of 700 and a Math score of 600 on one exam, then later receives a Reading score of 650 and the Math score of 700 on another exam, the university will take the Reading score (700) from the first exam and the Math score (700) from the second exam to create an ultimate superscore (1400) to be considered for the student’s admission.
At the moment, this is more common for the SAT than for the ACT, however, ACT superscoring is starting to gain popularity. For more information, check out The Troll, The Boy, The SAT & The ACT.