When you create an invoice, it is automatically associated with a specific project and client. If you have multiple projects with the same client, you can add additional projects to the same invoice, making it easier to manage your billing.

When you have multiple projects with the same client, you will see a button to 'Add Project' when editing your invoice:

Clicking this will open a dropdown to select other projects that you can add to the invoice. When you select a project, it will be added to your invoice as a new section where you can add invoice items and unbilled hours:

With this feature, all of your project billing is combined into a single invoice for them to pay.