the unique identifier of a view which already exists in the perspective. This will be used as a reference point for placement of the view. The relationship between these two views is defined by relationship. Ignored if relationship is "fast".Previously, I had thought that meant strictly that only a "view id" (including the ID for the editor area) could be used here - a thought that is reinforced by the tooling which lets you select any declared view ID in scope, including the org.eclipse.ui.editorss ID.
The issue in my application was that I needed a folder in the perspective - which could only be declared by the perspective factory - to be populated with view instances (same ID, various secondary IDs) from a down-stream bundle. The perspective factory can't have a dependency on the contributing bundle, and copying the view ID string literal into the up-stream bundle was not a tolerable solution.
What I wanted was to be able to declare a page layout folder as a perspective extension, or otherwise solely in the plugin.xml files for the bundles but was unable to find a way to do this.
However, as a solution, I discovered that a new folder can be created in the perspective factory code and then the down-stream perspective extensions can reference the folder's ID as a view ID. Ah-ha!
Now the solution: