dont need right now.
if you are trying to buy a stock, put the movies symbol in the trading box, type in the quantity (any number from 1 to 50000 if its a movie, or 1 to 10000 if its an option/derivative, or type 'max' which is however many you can buy), make sure buy is selected in the menu, and click place order.
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limits are for buying stocks at certain prices, the glossary linked here, explains it (go to the L section for limit orders)
good luck