By default, client accounts do not have Trading Access and cannot sign into TWS or Web Trader. They are limited to Account Management where they can view Statements and Trade Confirmations, as well as perform account administration tasks. However, there are occasionally requests by advisors that a client be given the ability to trade their own account.
There is a feature that allows the Advisor to give Trading Access to his Clients. This can be configured by the Advisor in Account Management -> Trading Access -> Client Trading. The Client would then need to subscribe to market data if they desire and would be able to trade only his own account.