An audience segment represents a group(ing) within your target custom audience. A segment may contain one or more first-party, second-party, and third-party categories. For example, a segment may include users interested in purchasing one or more products or services, users with a specific geographic location or demographics, or any other data category available through our Oracle Data Cloud partnership.
When you open the 3rd Party Audience Builder, an empty segment is displayed. To be able to save the audience, have to add at least one category to a segment. You can add as many segments as you want by clicking Narrow Audience. You can exclude segments by clicking the Exclude tab and adding categories to that exclude segment.
You can learn more about Categories in the knowledge base.