The project file contains two types of information. The first is a list of components, diagrams, packages, and so on that constitute your project. The second is a list of properties that you have overridden at the project level.
The project file (<Project>.rpy) is a unit that can be checked into a configuration management archive, which means you can execute configuration management operations on this file just like any other unit. Because the .rpy file contains the latest list of top‑level units in your project, you typically want to check it out for changes whenever you plan to add new packages or components to the project. (Adding new elements to a package requires a checkout of the package file, not the .rpy file.) In addition, you need to check out the .rpy file if you plan on modifying the project‑level properties.
Because it is a unit that can be placed under configuration management, you can use IBM Rational Rhapsody DiffMerge on the .rpy file. For more information about Rational Rhapsody DiffMerge, see Developing in parallel with Rational Rhapsody DiffMerge. You can also save an .rpy project file so that it can be opened with an earlier version of Rational Rhapsody.