There are some consequences if you do not register for UN 3002. If you are not registered for 2 credit of co-op then you will no longer be a full-time student. This could impact your financial aid (loans or scholarships), visa status, or qualification for health insurance. During the summer, however, most students do not need to maintain full-time status. In this case whether or not to sign up for the co-op class is purely your choice.
For those students here on an F-1 visa, the Career Center recommends that you register for co-op credits during the summer as well. By registering as a co-op student you are considered a full-time student and will not be utilizing your practical training time.
You may use up to 6 credits of co-op UN 3002 towards your technical electives (per Departmental vote 10Feb2016). UN 3002, UN 3003, UN 3004, and UN 3005 are on the technical elective non-core list. Any additional co-op credits above 6 can count towards free electives.