It was introduced in Windows 7
That method only works using numbers 1,2,3,4,5,6,7,8,9,0.
More than 10 windows and you're out of luck.
The number applies to the current position on the Taskbar.
It's trickier than that though.

If you have 2 Word windows open, only the first one "counts"(i.e all of the Word windows are accessed by the same Task bar number).
If I had Word windows in positions 7 and 8, "Windows Logo Key +7" grants me access to both of them (8 has no effect).
If I then opened another program (e.g. VLC) it would be accessed by "Windows Logo Key +8" (even though you would apparently have nine buttons on your Taskbar).
Here is a post I wrote listing Windows Logo shortcuts.
http://www.sevenforums.com/856297-post1839.html
You want a given combination to always open the same program?
The best thing that I can think of at the moment is to pin the programs to the Taskbar.
They should always use the same combination.
For example in my picture (above):
- WLK+1 always opens "Computer Management".
- WLK+2 always opens "Control Panel".
- WLK+3 always opens "Windows Explorer".
- WLK+4 always opens "Notepad ++".
- WLK+5 always opens "Firefox Selection batch file".
You can drag the pinned Taskbar items and place them in any order you like.