To find your available Contacts Segments select "Segments" as your File to Flatten in your job and run the job once (Flatten Once).

Open up your flat file (Google Sheets, Excel Online or any text/CSV reader) and look at the segments and their IDs.

Copy the ID of the Segment you wish to use.

Return your job and select Contacts by Segment as your File to Flatten (or use a different Job in Flatly).

In the Advanced Settings in the Filter option, paste the Segment ID you copied to your clipboard and click Submit.