"GUAR" or "GTY" booking is to say that you are guaranteed the category listed or better. For example, if you are GTY cat 11, you pay the cat. 11 price, and you are guaranteed either a cat. 11 room or better for the same price.
Some people extend this to say that you are more apt to be upgraded at GTY status. Whether that is statistically proven or not, is another story!
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