You can wait for a second opinion on this (I am not Adobe), but it is my understanding, if you make a download purchase from Adobe, it is for Premiere Elements 11 specific to your computer operating system of Windows OR Mac and has one serial number.
On the other hand, if the purchase is made via the boxed version route, you get one disc for Windows installation and one disc for Mac installation under the one serial number.
Tony is exactly right.
You are allowed, by the way, two installations of the program on two separate computers, if you own both of them. I have one installed on my Windows machine and one installed on my Mac.
>download Premiere Elements on both my PC and Mac with one disc
If you will go to the first message in http://forums.adobe.com/message/2276578 there is a link to a glossary of terms... so you don't confuse people
Downloading is what you do to save a file from the Internet to your computer... Installing is what you do to get a program "installed" to run