4110.1 General Policy
0000-0999 Developmental Precollege-level courses.
1000-4999 Courses intended for undergraduate credit only. The first digit indicates the general level of the course: 1 for a first-year course, 2 for a second-year course, 3 for a third-year course, 4 for a fourth-year course. Courses in this category that are acceptable for graduate credit carry alternative numbers in which the first digit only is changed to a 5 or a 6 according to the definitions below.
From Graduate School – Policy and Procedures Manual Section 4.10
5000-5999 Courses that carry graduate credit only in a major field different from that of the department offering the course. Most such courses will be alternate designations of undergraduate courses normally numbered 3000-3999
6000-6999 Courses that carry graduate credit in any major fields that have alternate designations of undergraduate courses normally numbered 4000-4999.
7000-7999 Courses intended for graduate students only
8000-9999 Courses intended for post-master's or doctoral-level work.