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G - Economy fare, restricted. Often used for group travel.
K - Economy fare, restricted, changeable for a fee.

X - Economy award. (no upgrade available INTO Eco, unless you can now buy tickets for the baggage hold)

Also, it might be worth mentioning the mileage levels:

German Domestic:

E L T U - 125 miles per intra-Germany sector
G H K M Q S V W - 500 miles per intra-Germany sector
B Y - 750 miles per intra-Germany sector
C D Z - 1500 miles per intra-Europe sector (Business)

Continental:

E L T U - 125 miles per intra-Europe sector
G H K M Q S V W - 750 miles per intra-Europe sector
B Y - 1250 miles per intra-Europe sector
C D Z - 2000 miles per intra-Europe sector (Business)

Intercontinental:

S W - 50% distance miles, minimum 500
G H K M Q V - 100% distance miles, minimum 500
B Y - 150% distance miles, minimum 750
C D Z - 200% distance miles, minimum 1000 (Business)
A F - 300% distance miles, minimum 1500 (First)

Values/classes taken from www.meilenrechner.de.