When using the Shipment Tracking - Unified API and not filling in the "service" parameter, you could call to track a shipment ID and receive the history of another shipment ID in the body response. We always recommend you fill in the "service" parameter if known to obtain better results.
The Shipment Tracking - Unified API is connected to multiple sources and databases; it has configured a logic where, depending on the structure of the shipment ID code, it will prefer to ask first one source before others if the "service" parameter is not filled. In the DHL Group, it's also common to have shipments that go through different DHL Divisions; each one of them will manage a different path of a shipment throughout its delivery process and may create additional internal tracking IDs that will provide information about this shipment and in another format.
An example of this matter:
There is a shipment that is being sent from Germany to France. While in Germany, it is processed by the Post & Parcel Germany division with the tracking code 00123456789101111111. Once the shipment has arrived in France, it's handled by the DHL Express division; they have already created their tracking code with the number 1098765432. When calling the Shipment Tracking - Unified API with the code 00123456789101111111 without filling in the "service" parameter, you obtain a response that refers to the tracking code 1098765432. This happened because the system identified that both codes referred to the same shipment and chose to show first the results from the DHL Express database.
If the same call is made with the tracking code 00123456789101111111 and specifying the "service" parameter as "parcel-de", then the body response will show the history of the shipment from the P&P view with the code 00123456789101111111.