Most of the examples in at the beginning of this guide use simple self-hosting. Resolving plug-in references using external plug-ins is simple, straightforward and fast. PDE discovers plug-ins and fragments in the target platform. Users can browse them, use them for references, extensions, extension points etc.
If simple self-hosting is used, external plug-ins are not part of the workspace and cannot be browsed in the Navigator or Package Explorer. For that reason, PDE provides a view that fills the gap.
The role of the Plug-ins view is to consistently show the plug-ins that will be passed to the run-time workbench when running or debugging. Plug-ins that are in the workspace are simply shown using project icons and no further browsing is possible (the intention was not to duplicate support available in Package Explorer and Navigator). However, external plug-ins have full hierarchy and their content can be browsed, editors can be opened on files (in read-only mode), files and folders can be copied into the clipboard and drag-and-drop can be used between the view and other compatible views. This way, Plug-ins becomes natural extension to Package Explorer.
When a workspace plug-in has a dependency on an external plug-in, its class path is computed by PDE by adding external JARs qualified using ECLIPSE_HOME variable. This variable is managed by PDE and points to the root of the target platform's install tree.