@sm2000 @PurpleTitan @billcsho Kinda let on this thread and tumble upon accidentally. However I would like to share our UIUC experience for my elder son last year.
First of all, for eng college if you declare any eng major as first choice then few other choices are mutually exclusive (E.g if you pick CS as first choice, you can’t pick CE as second choice and vice a versa).
2nd, if you denied in first choice, e.g. CS, if you don’t have 2nd choice like Electrical or other low tier eng., they may put you in PREP (PRe Engineering Program, it is a filter tunnel to engineering college that they started last year only), this is not same as Undeclared eng.
3rd, only way you can apply to undeclared eng is by selecting that first and only choice.
My son ended up with PREP (he fall short for CS) and he had just finished ICT transfer process to CS. However his friend who was in undeclared eng, got into CS acceptance last month.
I even have hard time to understand why UIUC has both, Undeclared eng and PREP.