Can anyone explain precisely the difference between these two REG.EXE v3 functions?

Here are the HELP examples:

Saves the hive MyApp to the file in the current folder

REG EXPORT HKLMSoftwareMyCoMyApp AppBkUp.reg
Exports all subkeys and values of the key MyApp to the file AppBkUp.reg

The only difference I can see is that I wouldn't know what to do with a .HIV file...!

Oh, and another question:
Can the file produced by REG EXPORT be double-clicked on and thus processed as a REG file by REGEDIT.EXE?
Or is the fact that the file:
a) is in UNICODE format
[img]/forums/images/smilies/cool.gif[/img] doesn't contain the string REGEDIT4 at the start
mean that this wouldn't work?
I would normally reply to me: "Why don't you try it?", but messing willy-nilly with the registry is not a Good Thing...