Festoon uses a tang or 'ear' index on the ends of their cords to prevent the lighter/small gauge cords from being used in the higher wattage tools accidentally. The heavier gauge cords don't have this prevention feature since they're rated for all the tools they can easily power up as compared to the thin cords.
I'm sure someone will link up the neat pictures of the different Plug-It sockets to show this better than my poor wording.. [wink]