Hi Mr Moosy, i think maybe the confusion here is because of the different test sheets/software etc layouts & the different way results are compiled. If you want to be 100% correct then a complete certificate should be filled in for the feed from wherever you connected to, to the new DB (What I still call a sub-main but I may get arrested now for such a faux-par) . Then a separate certificate for the final circuits etc.
From memory Easycert has the 'at origin' ze on the certs but on the test results page its ze at the DB (Zdb), Back to your OP, not doing any work like this for a long time now I suppose it can be difficult doing a true Ze 'at origin' in large factory complex's as where the metering is, is the origin, but it may not be possible there so wherever the main switch you turned off to isolate may have to do. The key is to make sure this is filled out on the test sheets and you have a good reason (such as a 24 hour production line) that you could/were not allowed to test at origin.
Tap off unit is a common term for busbar industrial kit. I suppose I only know that as I used to work for a large company and fitted loads. Haven't seen or touched one for 15 odd years now. Someone only doing domestics probably will never see or hear of it.