Switch Exchange
When you add a market data line to a Quote Monitor, you have the choice of directed exchange or SMART for the ticker's market data. For positions that display on the system-drive Portfolio page, this choice is made by TWS, and it may be the case that TWS uses an exchange to which you have no subscription. In this case there may be no data showing for certain positions on the Portfolio page.
The Switch Exchange button allows you to choose a subscribed exchange, enabling ticking data for the position from the Portfolio window.
Learn About Market Data in Trader Workstation at IBKR Campus