A panel is the base canvas that wraps onto your model. Panels are used to create PDFs.
When you set up your panel(s) remember to add at least 2mm bleed so that the edges of your design don't get cut off.
Panels are measured in points or pixels by a width and a height. So if your design is in millimeters or any other measurement you will need to convert it to points/pixels. You can do this easily using google or programs such as Adobe Illustrator.
You can have as many panels as you like. You just need to be able to match the panel sizes and names to what is on your 3D model. To find out what the names are on your model go to Babylon Sandbox and drag your model into the browser.
Now go to your Scene Explorer on the right hand side and select the Materials drop down. Here you will see the names of the panels in your model. This models panel is called Label.
It is important to to match the panel names exactly (case sensitive) to the material name on the 3D model. If you don't do this, the design will not show on your model.