Segmentation depends on the fields available in the report; therefore, each report can be segmented differently. A consolidated list of all reports and their respective segmentation fields is available
here. To segment a report via the API, all you need to do is to add the segmentation field to the report. In general, you can segment most reports on Date, DayOfWeek (Sunday, Monday etc.), Month & MonthOfYear (January, February etc.). However, segmentation on 15 days is not available. For more information on segmentation please refer to
this article.