If you have an immigrant visa case in process, there's a good chance that your file will, at some point, be handled by the National Visa Center (NVC). That's particularly likely if your case is one where you're either waiting for an available visa number (because you're not an "immediate relative" of a U.S. citizen or your "Priority Date" isn't yet current), or will be handled by an overseas U.S. consulate.
The NVC is sort of an intermediary, handling files during the transfer from initial petition approval by U.S. Citizenship and Immigration Services (USCIS) to an overseas U.S. consulate or embassy. The NVC has no decision-making power, but it plays an important administrative role.
Depending on the type of visa you qualify for, your file might be held at the NVC for several weeks up to several years. What if either the petitioner (who filed the initial visa petition, for example on Form I-130 or I-140) or the beneficiary (the intending immigrant) move during that time? For many type of immigrant applications, the beneficiary, at least, would submit Form AR-11 to USCIS.
But this DOES NOT WORK for cases at the NVC. USCIS won't pass the word along -- it's a different agency. Inconvenient though this is, you will need to communicate directly with the NVC. Here's the "National Visa Center Contact Information" for that purpose.