Functional Developer stores projects on disk in project files. Project files have the same name as the project, with the extension .HDP. Thus a project called Hello is stored in the file Hello.hdp. (Projects get their names in the New Project wizard or, if they were created by conversion from a LID file, get their name from the name of the library that the LID file defines.)
Project files are the files you select when you want to open a project or add it to another project as a subproject.
The development environment tools automatically save changes to project files. For example, if you add a new source file to a project, the change is saved to disk immediately.