IB allows account holders to designate and maintain their account as either a 'Margin' or 'Cash' type of account at the point of application. If one maintains a 'Margin' type account, the proceeds from closing stock sales are made available for trading effective with the sale transaction, however, the account holder may not withdraw those funds from the account until such time as settlement has taken place and IB has been credited with the proceeds from the clearinghouse (generally 3 business days).
If one maintains a 'Cash' type account, proceeds from closing stock sales are not available for trading or withdrawal until settlement has been completed. To provide otherwise would constitute an extension of credit which is prohibited from being offered to 'Cash' accounts.
Click here for information on upgrading from a 'Cash' type account to a 'Margin' type account.