Main reason bringing this to mind is PV installation voltage limit, which is 600V for domestic but can apply to many situations
There's no such classification as "commercial"; either it's domestic or it's not.
Where any part of the installation fits the "domestic" definition, that part must follow "domestic" requirements.
There are not many areas where different rules apply, the most common being RCDs
For example if I do in install on a large farm shed that has no domestic purpose what so ever but said shed is fed from a domestic premises, how do I treat this?
Or the other way around, when a commercial building was there first but someone decided to add a separate residence on site? If I did an installation on this commercial building can I go to 1000V? If it was on the residence would it be different?
In my view it comes down to deciding what the primary function of the premises as a whole is.
For a farm house not subdivided from the rest of the farm, the residential activity is secondary to the primary activity of farming.
Used to be that there would be separate supplies to house, woolshed, dairy shed, etc.
But since "fixed" line charges were introduced it has become more cost-effective to reduce the number of separate supplies that each get charged for at capacity, and use submains from a single supply.
So for a farm shed supplied from farm house; the house part of the installation is clearly domestic.
The shed has no residential function, so since farming is the underlying activity for the premises, can be treated as non-residential.
But a garden shed or garage on an urban / suburban property - where the primary function is domestic - the shed / garage would also be domestic.
That sort of decision tree works pretty well for eg RCDs, but your particular example of the 600 V limit for domestic PV arrays is a bit more complex.
I would judge this on where the alternative / supplementary supply is connected to the rest of the installation. If connected into a part of the installation that's residential, then that sets the rules. If connected into a non-residential part of the installation, then the limit doesn't apply.
So for similar installations with one having the MSB at the house, and another having MSB at a farm building; the location of MSB would set the rules for the PV array feeding into the mains-parallel capable inverter.
There is no definitive ruling that I am aware of; but to me it wouldn't make sense to judge it solely by the physical location of the array.
Notable that definition of 'domestic installation" allows for that classification to apply to only part of an installation. Same for "residential" in 2018 edition (which includes domestic). So it can't properly be argued that the location of MSB sets the classification for the entire installation.
Or even for all the final subcircuits originating at a particular switchboard.
Also as you're above the range of ELV, the array will need to be inspected as high risk PEW. So you need your assessment of how to treat it aligned with the Inspector's .