All full-time postgraduate students at Durham have to be a member of a College. Ustinov College only accepts postgraduate students and has wide variety of accommodation available for members.
To choose Ustinov College as your preferred college, put 'Ustinov College' as your first choice on the Postgraduate College Membership form. The college you choose will not in any way affect your chances of getting an offer from the University; that decision is made by your academic department.
You can apply for accommodation at the same time that you apply for college membership.