In the relationships matrix, the source and target can either be populated from a selected package in the browser or a custom search.
It would be useful to add an option to select a diagram so only the elements visible on this view and matching the selected type are used in the matrix.
Example: I have a diagram that I use to link specific BPMN tasks with UML use cases (business model -> analysis model traceability). I would like to see only the use cases (source) and tasks (target) from this diagram on my matrix.