“Command 6”

Is there anything special in MAC OS for the keystroke “Command 6”?   In the last century I assigned that keystroke to a particular macro in Microsoft Word.  Somewhere along the way I lost all of my macros in Word.  (It probably had something to do with an update to Word. ) I reconstructed most of my Word macros and reassigned the keyboard shortcuts to those macros.  Everything works. Yesterday I recreated another macro from way-back-when.  I assigned to it the same keystroke, “Command 6”, that it had in the last century.  Now the macro runs when I pull down the appropriate menu in Word, but the macro does not respond when I type “Command 6”.

I asked for advice in the Microsoft support forum.  The only response I have received so far is that maybe there is some intervening feature of the Mac OS.  

Is there any reason why “Command 6” would not work for the keyboard shortcut to which I assign it?