ShellExecute vs. Run

I was trying to run a script that runs another autoit script in the middle of it and learned that I need to use ShellExecuteWait(".\AutoIt.au3",".\childscript.au3") instead of RunWait with the same parameters.

I tried searching the forum for the difference between ShellExecute and Run and found this thread:

http://www.autoitscript.com/forum/topic/148548-run-and-shellexecute/page__hl__shellexecute#entry1056688


However, it doesn't explain why RunWait doesn't work for running child scripts and the help file doesn't really explain the differences either.  So what's the difference?

Thanks.