Once you have put in a specific course to look for and press the [Search] button, the page will display all open classes for that specific course, provided you left the "Show Open Classes Only" box checked. If you unchecked the box you will see all of the sections, open and closed, for that course.
When the search is complete, you will see the first three open sections that are available. To view all of the sections at once, click the "View All Sections" link in the blue bar at the top. You will see a blue link after the "Section" for a particular class. The four-digit number in the link is the section number, followed by either "LEC" for lecture and "DIS" for discussion. The number in the parenthesis is the class number you use to search for a specific section. Each section has a unique class number.
There are courses, such as most Math courses, that require a lecture (LEC) and a discussion (DIS) or lab (LAB) section. The classes are associated by number; if the lecture section is 1001, the discussions that go with that lecture begin with 11 (e.g. 1101). If the lecture section is 1002, the discussions will begin with 12 (e.g. 1201).