Copied explanation from Microsoft official site: ** Solution Files (.sln and .suo)**
Visual Studio uses two file types (.sln and .suo) to store settings specific to solutions. These files, known collectively as solution files, provide Solution Explorer with the information it needs to display a graphical interface for managing your files. They allow you to concentrate on your projects and final goals rather than on the environment itself each time you return to your development tasks.
.sln: Visual Studio Solution
Organizes projects, project items and solution items into the solution by providing the environment with references to their locations on disk.
.suo Solution User Options
Records all of the options that you might associate with your solution so that each time you open it, it includes customizations that you have made.
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