And should I add it to the source code repository?
It is for state persistence for Schema View. The state of this view is saved locally, there is no need to add it to source code.
The SQL Server project type has a special Schema View window and an extra Data menu item.
Also the Solution Explorer gets two extra buttons: Database Schema View and Reanalyze Project.
When the Schema View is open, 4 menu items under Data|Schema View are activated with the same functionality as 4 of the buttons in the Schema View window (a rather strange button in the Schema View window is the Solution Explorer button to return to the Solution Explorer..)
Note e.g. the External elements and Built in elements menu items in the Schema View window.
When you select Built in elements schema's of built-in db-roles become visible (e.g. db_owner).
When you select External elements schema's defined in different projects may become visible.
Such settings are saved in the schemaview file.
来源：https://stackoverflow.com/questions/3937160/in-the-new-visual-studio-2010-sql-server-project-type-what-is-the-dbproj-schem