I found this tutorial that shows how to build an AdHoc IPA for TestFlight. But there is also a TestFlight SDK, and I don’t understand if I need it and why I need it to easily distribute the beta test.
Can someone explain?
You do not need. You can upload any valid ipa to your website using this page: https://testflightapp.com/dashboard/builds/add/
If you have problems with your ipa, they have help here: http://help.testflightapp.com/customer/portal/articles/402782-how-to-create-an-ipa-xcode-4
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