This article explains the Targeting Segments Dimension in the Report Builder.
The 'Targeting Segments' dimension in the Report Builder (See Create Reports with Report Builder for more information) is limited to IAS customers who have purchased targeting profiles.
When you use the 'Targeting Segments' dimension, you can only use the Campaign, Media partner, DSP, DSP Campaign, Segment, and Team Dimensions in the Report Builder. With the 'Targeting Segments' dimension, you can filter to show select targeting segments.
Additionally, when you use 'Targeting Segments' dimension, you are limited to the following metrics in the Report Builder:
Eligible Ads for Viewable Measurement
Not Viewable Ads
Not Viewable Rate
Total Tracked Ads