Ok. I got it. Thanks!Let's use Windows 7 as an example, there are several editions: Home Basic, Home Premium, Professional, Ultimate and Enterprise. EACH of the those editions you get up to 10 product keys to use.
Let's say I have 10 product keys for Professional. I am using product key #1 for my computer. My son wants to update his desktop to Windows 7 Professional. Being the loving, generous father that I am... I give him Product Key #7. It's his now and I will no longer be able to use it, but I still have product keys 2-6 and 8-10 which are not activated and for my use.