Script load.sql first copies from Underlying to Symbol, giving us an initial set of symbols. Keep in mind that a symbol is an abstraction, not just a string like SUNW, or JAVA. Those are names, and a symbol has in addition a type, e.g., stock or future; and a home, since different exchanges may use the same name for different symbols, that is underlying products.
Although Symbol now has an initial population originally derived from: the primary currencies supported by IB, USD, AUD, CAD, CHF, EUR, GBP, HKD, JPY, MXN, and SEK; miscellaneous index, commodity, and paper names from Miscellany; and the stocks from, you guessed it, Stock, currently mostly NYSE and NASDAQ, and some AMEX -- still, there are any number of possible derived products not yet in symbol. In particular, for the entries in Miscellany, we may want to use the same name for related indices, futures, and options.