It deals with the types of clubs you can carry in your bag. The following rule has been copied and pasted from the USGA site and the rules section:
Rule 4-4
Types of Clubs Selected for the Stipulated Round
Q. May a player carry more than one of a certain type of club (e.g. more than one putter or driver) or carry some right-handed and some left-handed clubs?
A. The player must select not more than 14 conforming clubs (see Rules 4-1 and 4-4) for use during his stipulated round.
He may select any combination of conforming clubs i.e. he is not limited to only one putter or only right-handed or left-handed clubs.
Currently we cannot break up the full iron set for a round. We also cannot carry more than one driver or putter. The USGA rule clearly states that the player can carry any combination of conforming clubs as long as the number of clubs does not exceed 14.
A couple of examples. Maybe I'm playing one of the shorter courses and I know I may not need a 3 wood. I might chose to shelve the 3 wood and carry a 2nd driver. So perhaps I have the lower trajectory R1 to hit shots into the wind, and the higher trajectory Big Bertha Alpha to get over some trees or cut corners a bit better.
Perhaps a more practical example would be the following:
If I could break up the iron set, I might chose to carry one of the new Adams hybrids and drop the 3 iron for the round. I also might drop the 120 pitching wedge and carry a 4th Cleveland wedge (the 115 yd Cleveland).
The point is, is that USGA rules allow you to have 14 clubs of any type you chose. Theoretically you could go out for a round with 14 3 irons if you wanted to.
So if a WGT monitor reads this post, please suggest that the game be enhanced to allow this. Not sure how much of a programming change it would be, but if it's able to be done it would make the game that much more flexible and interesting :)