Most of the time, you'll be creating Audiences with Audience Builder, which allows a very simple way to create complex selections from people in your app.
Inside, you can choose from a combination of User Attributes, Events and Location Data. On the righthand side, a summary and an estimate of the number of users that will be in that audience.
The Notifications API allows you to create on-the-fly audiences (in other words, audiences that are not defined in Audience Builder) when sending a notification. These audiences are transient and don't persist between requests. To learn more, see the examples in the Notifications API reference.
If you are capturing User IDs in the app, you can upload a list of User IDs as a CSV (one ID per row) to create an audience segment. After you upload the CSV, we match CSV users with your installed base and we add a flag to a user profile if there's a match. This way you can target all users who belong to a specific upload or to subsequent uploads.
You can easily update an imported audience by always specifying the same audience name. In this case, Sailthru Mobile will append new users to the last upload.