The rules sheet says there's a "special mark in the bottom left corners" of certain cards "to make sorting easier" when you want to play the simplified version of Fluxx without the Zombies.
This info is on the wiki in two spots within the Zombie Fluxx page:
http://rabbits.continuation.org/wiki/Zombie_Fluxx#Specifics (the 6th bullet point down)
http://rabbits.continuation.org/wiki/Zombie_Fluxx#Playing_Without_Zombies (card list that has this special mark is here)
Hope that helps!
Carol
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