Answer
Target PURCHASE_RECEIPT_TAB — registering an arrival on that screen creates a new row in that table, so an insert-type custom event on it fires exactly when a line is received, even though from the user's perspective in Register Arrivals it looks like the line is disappearing rather than a new record being created. The exact event configuration (insert vs. update, which columns to watch) still depends on precisely what you're trying to trigger on, but PURCHASE_RECEIPT_TAB is the table to build it against.