Hope you had nice Christmas.
This really is a great questions and I totally agree there should be some kind of ‘clear’ rules that specifies the essential tests that cannot be missed. maybe the IET just assume that fold will know because each field on the back page is used to verify certain things. For example:
To calculate disconnection time and Adiabatic you need install method, wiring type, Zs, voltage cps size etc.
If any of these fields are missing you can’t really confirm the installation is safe.
I would personally say any field that is complete using the engineers test equipment and physical sight, and obviously the address fields are essential. The rest can be assumed.
I guess the answer to the question is every field that you need to verify that the whole installation is safe for continued use.