Anupam,
Look at it this way, if your field value has "number of cars" in a City and the normalizing field has "population" of the City, the ratio of these two numbers is used to determine the symbol size and would represent "number of cars per person". If the normalization field is not set or provided the symbol would be based exclusively on "number of cars".
Another example if the field value was "total personal income" and the normalization field was "number of households" the symbol size or attributes would be determined on "personal income per household" or personal income divided by number of households.
Simply, the field value is divided by the normalization value giving a ratio for use.
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