This is a post by Mike Gayette on PTC site.
For sheet metal parts, it can be important to understand how your 3D model appears in a flattened state in order to precisely understand material needs and dimensions.
Flat pattern representations allow you to create a simplified representation of a sheet metal part in the flattened state. The master representation can remain in the formed, bent state, while the flat pattern representation resides in a combined view. Then when you display Combined Views, you can toggle between views and see how the flat piece is affected by modifications to the master (bent) representation.
Here’s what you need to know to get started: