To simplify the answer......
The difference is that each Windows ISO file contains a "key file" corresponding to the Windows version of the ISO file.
Example : A Windows Ultimate ISO file will have an Ultimate key file. One has to have an Ultimate product key to install the ISO.
There is a software which will remove that "key file" restriction so that the ISO becomes an "universal" ISO file which can be used for any product key one has.
I am not sure about the forum policy regarding the use of the software mentioned. So, I'll stop here.
The important point is....... ISO file is useless unless you have a valid product key.