The Compatibility Layer is a set of bundles that have been forked from the Eclipse 3.x stream to allow plug-ins written against the 3.x APIs to run unchanged on an Eclipse 4.x SDK.
The following bundles have been forked from the 3.x stream:
The only interesting bundle here is the org.eclipse.ui.workbench bundle as the org.eclipse.platform and org.eclipse.sdk bundles do not actually export any API.
The internal implementations of interfaces like IWorkbenchWindow and IWorkbenchPage from the org.eclipse.ui.workbench bundle have been replaced by new implementations that calls out to Eclipse 4 services and APIs.
When to Use
As the Compatibility Layer was written to provide a way for Eclipse 4 applications to host Eclipse 3.x plug-ins, its target audience will be primarily geared towards developers of Eclipse 4 applications that have 3.x editors or views that they wish add to their application. Unfortunately, this use case is not well-supported at the moment.
Developers have however used the Compatibility Layer to run Eclipse 3.x RCP applications on top of Eclipse 4 technologies, effectively empowering them with the ability to style their application as well as the ability to open views within the "editor area".