Useful Trivia: The first number lets you know what LSC campus the class is at! UP class sections all start with 6 because we're the sixth campus. North Harris classes all start with 1 since they were the original campus.
Your course section number identifies your specific class from all the other courses. They are reused from semester-to-semester, so there's always, for example, a HIST 1301 section 6001, even if all the other details change.
You can find your section number for your current classes in D2L, provided in between the course (e.g. ARTS-1301) and the course name (Art Appreciation).
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When you're in the course catalog looking to register for classes, the section number is labeled:
You may notice that each class a unique number assigned to it, as well, but those aren't the section numbers (or the course numbers, for that matter).