By default, your portfolio shows performance and activities for all your exchanges. You can always select and deselect any number of linked exchanges in Portfolio view. To do so, please, click on any exchange in the top right corner of your Portfolio graph, this will change the entire data to that particular exchange.
When you switch your view to a single exchange, the information on your Portfolio will present data for that exchange only. You can use this option if you want to analyze your performance or export data for selected exchange. Please note, when you choose or have a single exchange, your balance chart and portfolio graph will automatically switch sort by list to coins - where you can see the overall performance for each of your coin you have on that exchange. Your Holdings, Sold and Transactions tab will also be changed to show you the data for this exchange. This can be particularly useful if you need to export data from a specific exchange.
By selecting two or more exchanges at the same time, your the portfolio will adjust trading data for these exchange only. This time, your balance chart will give you information about where are most of your funds allocated. While Holding, Sold and Transactions tab will show you the records only for selected markets.