Restrictions pertain to major, credit-based class standing, and prerequisites. Some courses may be limited to specific credit-based standing or specific majors for the purpose of enrollment; however, the intention may not be to limit the other classes or majors entirely. If you are interested in taking a course that does not include your particular credit-based class grouping or major, contact the instructor to request an override/waiver of the restriction. The instructor may or may not be able to grant you an override, depending on whom he or she has targeted to take that particular course. For pedagogical reasons, some may be more restrictive than others, but it doesn't hurt to ask.
Also, instructors may be particular as to when they might grant an override of restrictions. If you are a senior trying to register for a 100 or 200-level course, the instructor knows that if he or she grants you an override you are essentially guaranteed a seat in the class that is geared toward students who will register on the latter days of registration. If this is the case, an instructor may choose to grant you an override on the last day of registration, or he or she may instruct you to come back with an Add card during the Add/Drop period that begins the following week, if seats are still available in the course. If you are lucky, the instructor will grant you an override before registration begins!
Check the course restrictions for courses you are interested in taking by clicking on the 5-digit CRN on the left of the course schedule, then on the underlined course title to review the course set up details such as major, credit-based class standing, prerequisite restrictions for gaining access to the course.
When you register for courses for which you do NOT meet the specified restrictions, you will get a specific error message telling you exactly what you do not meet: Prerequisites, Major, or Class restrictions. Without your attempting to resolve the issue or being granted an override/waiver of these restrictions, the error message will not resolve itself no matter how many times you attempt to register for the course. You must take action to resolve the error and then try to re-register for the course.