This article is written for "Purple DS Suite". Further Information can be found at Apple Support . 

When you first open a Purple DS app (Enterprise Developer) that you've installed with Purple DS Manager, you see a notification that the developer of the app is not trusted on your device. The message is shown whenever you try to install an app from a different source than the Apple App Store. You can dismiss this message, but then you can not open the app. This notification only appears on Apple devices.

After you dismiss this message, you can establish trust for the app developer. Tap  "Settings"> "General"> "Device Management" or "Profiles & Device Management" .  Under the "Enterprise App" heading, you see a profile for the developer. Tap the name of the developer profile under the Enterprise App. 

Then you see a prompt to confirm your choice. After you trust this profile, you can manually install other apps from the same developer (e.g. from the Purple DS Manager) and open them immediately. This developer remains trusted until you use the "Delete App" button.