The rules...if you want to know them...are all posted at the Microsoft website.
Just search for "EULA for OEM license".
But...I don't think you want to know "the rules".
There are any number of posts re licensing, etc. available in the archives of this and (probably) every other computer help forum.
I guess that I don't see what is there to misunderstand or not understand, especially considering the ability of Platypus to lay it out for anyone to read.
If you want to know something other than "the rules"...just ask your question and we all will be glad to chime in with our personal opinions. Many of us have OEM licenses for XP.
Edit: Somewhat related recent post, http://www.bleepingcomputer.com/forums/t/177121/can-i-use-the-windowsxp-disc-i-currently-have/
Edited by hamluis, 31 October 2008 - 01:23 PM.