With a click on New platform you can add predefined machine platforms to the project tree. This feature is available in the toolbar, then choose the tabsheet default machines. The listed platforms contain the exact measurements of actual machines types and allow you to pack multiple platforms at the same time. Please see a detailed list in the machine library menu.
You can also add new platforms and define their measurements yourself. Just open the New platform dialog and stay in the tabsheet Custom machines. Enter a machine name and its details and choose if you want to add, remove or change this new platform.