You can use tags to split the accounting into different parts allowing you to see the profit and loss for each part. For instance, if you have two large clients you can create a tag for each one and assign incomes and expenses to each client.
When you've created at least one tag you can assign tags to individual rows in entries, and you can select a tag for customer invoices and for supplier invoices.
To see the profit/loss for a single tag, select 'Profit/loss statement' from the sidebar and choose the tag in the popup menu at the bottom.