Testflight is put on the shelves, also known as tf putting on the shelves or tf signing. Testflight is Apple's official internal testing distribution channel, which can be simply regarded as Apple's internal testing application store, which is much easier and simpler than App Store.
Enterprise signature: that is, using Apple's enterprise certificate. By signing the application, it has the ability to install directly without shelving and auditing, jailbreaking.
Super-signature: Real machine testing is realized by adding the udid of Apple device to the personal developer account. Supersignature can complete this process automatically.
The above three methods, operation steps and installation steps of ipa installation can be seen in this article, which is very detailed.
There are three ways for Apple to install ipa: enterprise signature, super signature and tf signature.