Exercise 2: Diagnosing a Beta Tester's Failed Install — Possible Solution
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The chapter's own real device-limit table draws a sharp distinction:
Development and Ad Hoc provisioning profiles are genuinely limited to
a specific, named list of registered device UDIDs, while App Store and
Enterprise profiles carry no such limit at all. A beta build - the
kind sent directly to a tester rather than distributed through the
App Store - is almost always signed with an Ad Hoc profile.

Given that, the single most likely cause of the described failure is
that the tester's own real iPhone was never actually added to the Ad
Hoc profile's device list in the first place. The build itself can be
signed completely correctly - a valid certificate, a valid App ID, a
genuinely working provisioning profile - and it will still refuse to
install on that one specific device, because the profile explicitly
does not authorize it.

The real fix follows directly from the diagnosis: register the
tester's own device UDID with the Apple Developer account, regenerate
the Ad Hoc provisioning profile so it now includes that UDID, and
rebuild the app using the updated profile. Simply resending the exact
same, already-built IPA file will not fix anything, since the profile
embedded inside that build is the one still missing the device.

ANSWER: The most likely cause is that the tester's own device was
never added to the Ad Hoc profile's real, specific list of authorized
devices - Ad Hoc profiles enforce a device limit that App Store
profiles don't. The fix is to register the device's UDID, regenerate
the Ad Hoc profile to include it, and rebuild the app with the updated
profile, rather than resending the existing build.

WHY THIS WORKS AS AN ANSWER
------------------------------
This correctly identifies the device-registration gap as the real
cause (rather than a signing error) using the chapter's own established
distinction, and explains why rebuilding - not just resending - is the
actual fix.