What is .database file used for in SSIS solution

Quoting MSDN's page on Integration Services Projects

Files in Integration Services Projects

When you add a new or an existing Integration Services project to a solution, Business Intelligence Development Studio creates project files that have the extensions .dtproj and .dtproj.user and .database.

• The *.dtproj file contains information about project configurations and items such as data sources and packages.
• The *.dtproj.user file contains information about your preferences for working with the project.
• The *.database file contains information that Business Intelligence Development Studio requires to open the Integration Services project.