Module 5 Economy allows you and the other players to buy and sell shares in aerospace companies whose value is tracked in a matrix called the stock market. The players themselves are the customers for these companies, and with more customers for a customer service, the more capital the company can purchase and the higher their share value.
If you own the most shares in a company, you are its chairman and decide if dividends are paid out to the shareholders or instead the funds are reinvested into company capital allowing them to have cheaper services. Players buy shares in companies that are likely to rise in value and sell them when their cash could be better used elsewhere.
Because shares are so easily bought and dumped, and they retain value after an Aqua crash, holding shares is generally superior to holding Aqua.
