a) does dynamic range 1000:1 same as 1 order of magnitude?
1 OoM is like shifting the decimal point to the left/right by one digit
(divide/multiply by 10), so 1000:1 = 3 OoM
b) what is maximum dynamic range that can exist in real world?
Very much depends on your scene: The sun has a luminance of 10^9 cd/m2,
and 'no light at all' would be 0 cd/m2.
c) what dynamic range can human eye see?
Please see WebHDR:
d) what do we mean with cd/(m^2) ? where m^2: m=base, 2 = exponent
candela per meter square is the unit of luminance. 2 is not an exponent,
just a simple square.
See SynthLight, chapter one for more info on photometric units: