Lumen is a personal video player for Plex libraries. It is not affiliated with Plex Inc.
Nothing. Lumen has no analytics, advertising or tracking, and the developer receives no data about you or what you watch.
When you sign in, Lumen talks to plex.tv to sign you in and find your servers, and then to the Plex server you choose. To play video in full quality it uses a companion service run by the owner of that server, which checks your Plex sign-in with the server before sending anything. Your Plex sign-in stays on your device, in the system keychain.
Downloads, their artwork, your chosen server and your quality setting are stored only on your device. Deleting the app removes them. Watch progress is sent to your Plex server so it stays in sync with your other Plex apps.
Demo mode plays public sample videos from Apple and does not sign in or connect to any Plex server.
Questions: use the feedback option in TestFlight.