What term is used to describe the design file that Fusion 360 opens by default when starting anew?

The term used to describe the design file that Fusion 360 opens by default when starting anew is "Untitled Design File." When a user opens Fusion 360 and begins a new project without initially saving it, the software creates a temporary file that is labeled as "Untitled." This allows users to start their design work immediately without the need to specify a file name or location until they are ready to save their work.

This approach streamlines the design process, especially for new users who may still be familiarizing themselves with the software environment. As users progress in their design, they can later assign a name and save the file to an appropriate location. Other options like Prototype File, Template File, and Initial Project File are not the correct terminology associated with the first design file opened in Fusion 360. These terms refer to other specific types of files or organizational structures within the software that serve different purposes.