This section describes the structure of project files.
A project may be an independent project, entirely defined by a single project file. Any Ada source file in an independent project depends only on the predefined library and other Ada source files in the same project.
A project may also depend on other projects, in either or both of the following ways:
The dependence relation is a directed acyclic graph (the subgraph reflecting the “extends” relation is a tree).
A project's immediate sources are the source files directly defined by that project, either implicitly by residing in the project file's directory, or explicitly through any of the source-related attributes described below. More generally, a project proj's sources are the immediate sources of proj together with the immediate sources (unless overridden) of any project on which proj depends (either directly or indirectly).