Segment statistics is accessible for each segment by clicking on the button as shown in the screenshot below.
Segments statistics allows you to capture the audience dynamics by day and differentiate the data by Opt-In, Opt-Out, Delta functions.
You can compare audience in size with other segments under the same category.
Audience by Source displays audience composition by sources : Smart Segment Builder, Segment Builder (Ad Serving and Site Tracking), Server-to-Server (S2S), Profiling Pixel (PP), Cross-Device (XD), Audience Extension (AE), IP segmentation, Manual upload. Total amount of UUIDs is dispayed in Audience line, while % of Total indicates percentage by source.
Also, you can see how many UUIDs belong to Browser users and how many to Mobile and/or Tablet users (Advertiser ID).
Audience totals allows you to evaluate audience dynamics by day.