It depends on the type of meeting you have created. 

In Planned Agenda meetings a moderator can manually sort the agenda. In these meetings, you can either drag and drop the topics into the desired order, or you can use the dot menu on the right of each topic to move the topic. A One-on-one meeting is similar, but both the moderator and attendee can manually order the agenda. 

In a Collaborative Meeting, the topics are sorted by vote count. This helps ensure that the most valued topics are discussed first and the least valuable topics get dropped if you run out of time. As a moderator, you can set the number of votes each attendee gets. You can preview the agenda order before the meeting starts by turning on the "Preview Discussion Order" toggle above the agenda.

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