Answer
There's no single documented mapping table, because the exact variable name/case can depend on how the event was defined — the reliable way to find out is to use Workflow Observations (watch mode) to trigger the workflow and see exactly what variable names and values actually arrive, rather than guessing from the event's field names. If you're not sure where the failure is, temporarily strip the workflow down to just a Start and End event so it runs without error, then rebuild it step by step while watching the observations to see when the value you expect actually appears.