UIUC (including engineering) does not require, recommend, or consider subject tests or recommendation letters for admission. If you send subject test scores or recommendation letters, they will not be provided to those who make the admission decisions. As to SAT and ACT, they do not formally superscore but instead use that test with highest composite except that some of the colleges or departments may consider a higher section score from a second test. Engineering uses the test with the highest composite but will consider a higher math score from another test.