META-INF/MANIFEST.MF: The MANIFEST.MF file, also known as the OSGi manifest, describes the plug-in's name, version, and dependencies. Double-clicking on it will open a custom editor, which shows the information entered in the wizards; or it can be opened in a standard text editor. The manifest follows standard Java conventions; line continuations are represented by a newline followed by a single space character, and the file must end with a newline.

plugin.xml: The plugin.xml file declares what extensions the plug-in provides to the Eclipse runtime. Not all plug-ins need a plugin.xml file; headless (non-UI) plug-ins often don't need to have one. Extension points will be covered in more detail later; but the sample project creates an extension for the commands, handlers, bindings, and menus' extension points. Text labels for the commands/actions/menus are represented declaratively in the plugin.xml file, rather than programmatically; this allows Eclipse to show the menu before needing to load or execute any code.

build.properties: The build.properties file is used by PDE at development time and at build time. Generally it can be ignored, but if resources are added that need to be made available to the plug-in (such as images, properties files, HTML content and more), then an entry must be added here as otherwise it won't be found. Generally the easiest way to do this is by going to the Build tab of the build.properties file, which will gives a tree-like view of the project's contents. This file is an archaic hangover from the days of Ant builds, and is generally useless when using more up-to-date builds such as Maven Tycho, which will be covered in Chapter 12, Automated Builds with Tycho.