If your account has insufficient cash, then debit of fees, interest, and/or taxes could result in a negative cash position. This may put the account at risk of liquidations, as described in the terms and conditions of the Customer Agreement.
Additionally, clients with a cash account can use the proceeds from the sale of a security to purchase a different security under the condition that the second security is held until settlement of the initial sale. If the client sells the second security prior to settlement of the initial trade, they will be in violation of the Free Riding Rule and their account will be locked for 90 days from utilizing this exception.
Account holders who wish to have access to unsettled funds prior to the settlement day may do so by electing an account type of 'Margin'. Under this account type unsettled funds may be used for trading purposes but may not be withdrawn until settlement. Account holders maintaining a 'Cash' account may request an upgrade to a 'Margin' type account by logging in to Client Portal and selecting the User menu (head and shoulders icon in the top right corner) followed by Settings. Under Account Settings find the Account Configuration section and click Account Type. Then click on the word Cash and select Margin. Next, answer all of the presented questions. A review of the request can take a few days.
You can find details of the procedure in the Client Portal Users' Guide.