Answer
Two ways to find this: in the Object Connections page itself, search for LU Return% to see the currently-configured connections for RMA/RMA Line-related logical units; or, in the Technical Documentation, navigate to the Object Connections page for the relevant logical unit and scroll to its Available Services section, which lists every object type that's technically eligible to be connected — not just the ones already configured — giving you the full picture of what's possible versus what's currently set up.