Why does my Sendle parcel have two different tracking numbers?

Sendle was an aggregator that handed parcels to partner carriers, so a shipment often carries the Sendle reference number (starting with "S") plus a separate tracking number from the last-mile carrier such as USPS, Couriers Please, Aramex, or DHL. Both numbers refer to the same parcel; the partner number usually shows the most detailed local scans.