Answer
APPROVAL_ROUTE_REJECTED fires specifically for approval processes tied to Documents, while STEP_REJECTED_NODOC fires for the same kind of rejection on every OTHER, non-Document object type that has an approval process configured. Approval processes themselves can either be a built-in system-defined feature on certain objects, like Engineering Part Revision or Project Item, which have a dedicated Approval tab, or configured generically on essentially any other object via Object Connections — the '_NODOC' naming on the second event reflects that it covers approval-enabled objects other than the Document-specific case that APPROVAL_ROUTE_REJECTED is dedicated to.