Elements, both in Fire and Visual Studio, works with projects inside of a Solution. You can think of a Solution as a container for one or more related projects, often called a project group or a workspace in other IDEs.

Fire and Visual Studio will always open a solution, not a project – even if that solution only contains a single project. When you open a .elements (or legacy .oxygene) project file directly, the IDE will check if there's a solution file with the same name next to it, or else automatically create a solution for you.

Solution files use the .sln file extension, and just like Elements projects files, they are compatible between Fire and Visual Studio (the file format is defined by Microsoft for Visual Studio, and Fire honors the same format). They are plain text files, but you will not normally want to edit them directly unless you know what you are doing.