The US petitioner receives the NOA2 notice of Action Approval notice from the United States Citizenship and Immigration Services (USCIS). The US petitioner is given a USCIS case number that they can use to track the status of the i-129f package as it processed and eventually makes it’s way to National Visa Center.
Once the USCIS sends the package to the NVC, your case is given a NEW US State Department Case Number. We think this is because USCIS is with Department of Homeland Security and NVC is under the US State Department. These are two huge departments within the US government that handle many different types of cases in their own way.