I do not see the problem, the input can handle up to 450V peak or DC before damage, and the meter can show readings up to 303V, i.e. from 303 to 450 the meter will not show correct (It will probably either show 303 or overload), but the meter will not be damaged.
Yep, I understand the second bit. I just wasn't sure why they left out the +/- sign in front of 450V whereas they used it everywhere else. Surely probing things back to front wouldn't result in a blown meter! Would happen so often!
The meter is rated to show +/- voltage, i.e. the sign is already implied in the range.
The isolation between the electronic and the box (i.e. Earth Ground) can handle up to 500V and that is the same from either input terminal.
This I still don't understand.
Not all voltages are measured relative to ground, sometimes you need to measure a voltage on top of another voltage and in these cases the total voltage must not be above 500V, i.e. you cannot measure a 5V supply is on top of a 600V supply like: 0V / 600V / 605V, but you could if the connections was: -600V / 0V / 5V (It is the same voltages, but ground is connected different).
All meters has this rating, on handheld it is often the same as maximum input voltage, but bench meters are different.