In terms of a Motorbike…… My understanding is if the frame has previously been registered in Australia and has a number stamped on the neck along with a compliance plate. As long as those two items are intact it is deemed a modified vehicle.
A modified vehicle must comply with the original year of compliances ADRs. Usually the year of manufacture but not always. I got caught here as my bike is a 1976 according to the manufactures list of numbers, however it is listed as a 1978 according to it's compliance plate. Maybe we got the old superseded models dumped on Aussie showrooms back in the 70's.
If you have a complete new frame then it is deemed and ICV and most comply with the year of COMPLETION and TESTINGS ADRs! So you could get caught out if your build takes too long and ADRs get changed.
There seems to be a lot of misinformation circulating on what is or isn't required what is legal and not so much.
The reasons being are partly due to the information changing over time, being only relevant in certain circumstances and having exemptions. Being open to authorities interpretations, or being written for cars etc and attempting to be applied to bikes. Also the boys in blue only get something like a 3hr training session on this so often can't be expected to know the specifics in depth. They have a lot of law to try and remember.
Best advice when dealing with the police, RWC assessor, or any other relevant body is to know your shit better than they do and have printed laminated copies of the rules specific to your build that maybe questioned tucked up in your tool roll. At the first sign of debate rip out said papers and end the debate on the spot and put whoever is questioning the legitimacy of your mods back in their box.