"What's that?" I wondered to myself. I searched and searched long and hard hours, in the rain, in the snow, and near mountains****. Naturally, the first article that popped up was an MSDN article: Solution Items. There weren't very many articles about it, which means it was a good candidate for this blog****.
In a lot of cases, there are files that a developer might want to draw attention to a file and add it to a Solution that doesn't necessarily fit into the actual code, for instance README files or installation instructions. They are not source files, they are not project files, they are just ... miscellaneous files! But they are still important.
* For those who don't know, SSDT is just a shell of Visual Studio. Ergo, this information also applies to the full product.
** Which is weird because I spend 30 hours a week in SSDT.
*** It's not hard. I live in Denver and it was raining that day.
**** I try to only post about things that don't already have a good presence in the Internet Mindshare.