Once your upload is finished, it will appear in the targeting system for emails, reports, and queries, allowing you to include or exclude all activists uploaded in that file.
Each upload will appear in the actions section, along with petitions, forms, fundraisers, events, and other action types. You can choose it for include or exclude just like any other action.
A second entry for each uploaded file is also created, identifying just the new people to your list that were added in that upload. This allows you to, for example, send just new people a welcome email, while not sending that email to activists who were on your list previously.