The Open Web Site in Solution dialog box appears when you open a local Internet Information Services (IIS) Web site that contains other Web sites. A local Web site is an IIS Web application on your computer. Visual Web Developer reads IIS metabase information so that these Web sites are interpreted as distinct Web applications in Solution Explorer.
Open this Web site (this will close the current solution)
When you choose this option and click OK, the current solution closes and the selected Web site opens as a new Web site. You will be prompted to save any changes to the current Web site if there are changes that have not been saved.
Add the Web site to the current solution
When you choose this option and click OK, the selected Web site expands and is added to the existing solution.