@riceball123 - For continuing students, housing is on a first come first serve basis.
https://housing.uci.edu/apply/Housing_Process.html
"Offers will be made in application date/time order (according to applicant preferences) until all available spaces in the communities are filled. If we are not able to offer your highest ranked community, your second choice community will be offered (as long as space is available). If that community fills up, your third ranked community will be offered, etc. "
I am unclear of the process for new freshman, but I do recall that students with disabilities or special needs have their assignments done first. Completing the housing application by the date just guarantees that you have housing for the fall. Please call housing if you have further questions. 949-824-6811