@clive:
The Kenngruppen are the rotor settings visible through the slots on the surface of the Enigma.
While the general Enigma settings (rotor settings, internal rotor settings, Stecker settings) are changed only once a day (at 0:00 hrs) and stay unchanged for the remainder of the day, the outer settings of the Walzen (rotors) are changed at specified intervals.
There are always four Kenngruppen to each day. The first one is used from 0:00 to 06:00 hrs, the second from 06:00 to 12:00 and so forth.
This is yet another weakness of the German Enigma concept. Important messages had to be repeated several times by the BdU, cause some subs might be underwater by the time the message was sent.
So, the plaintext had to be encoded again, this time with a different set of Kenngruppen. Since no letter typed on an Enigma will ever be encoded into itself, and the two messages having the same wording, each letter at its respective place could be excluded to be at that place in the plaintext. This gave the decoders at Bletchley yet another "crib" as to what the message meant. It might even give Bletchley a clue as to a whole day's settings, if the text was reencoded with a new days's settings.
@Muscle: It doesn't mean to send documents around at all. The code book could be deposited as a .pdf file on a web site for all authorized to see. But allright, you just go and re-invent the wheel.
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