What does the number on the visual studio solution icon represent?

Just out of curiosity - what does the number represent? At first I thought it was simply the version/year, because I saw some 10's and some 8's (I have vs 2010 and 2008) but, then why do I have some 9's aswell?

Solution 1:

The numbers correspond to the internal version numbers of various editions of Visual Studio

http://en.wikipedia.org/wiki/Microsoft_Visual_Studio#History

Visual Studio 6.0 (1998)
Visual Studio .NET (2002) = version 7
Visual Studio .NET 2003 = version 7.1
Visual Studio 2005 = version 8
Visual Studio 2008 = version 9
Visual Studio 2010 = version 10
Visual Studio 2012 = version 11
Visual Studio 2013 = version 12
Visual Studio 2015 = version 14
Visual Studio 2017 = version 15
Visual Studio 2019 = version 16

The number on the icon indicates the version that the project or solution is compatible with.

Solution 2:

It's what version of Visual Studio the solution was created with. 10 is for Visual Studio 2010, 9 is for Visual Studio 2008, 8 was for 2005, and 7 was for 2003. You might seen an 8 sometimes with VS 2008 if you set it to target .Net 2.0 features only, but that usually means you found an older solution file somewhere.