I wanted to try the latest SE iOS app beta on iOS 9 (13A4325c).
After downloading v 220.127.116.11 via mobile safari, this modal dialog casts shade upon stack developers:
Have I missed a certificate to install?
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This is a new iOS 9 security feature. Enterprise signed apps that are not installed through MDM have a manual approval step required before the OS will launch the SE app that's signed for enterprise distribution instead of App Store distribution.
As proposed by the dialog, trust StackExchange to be a legit and trusted app provider.
When the profile is selected:
Trusting the app: