The letter says Visa number is available if you don't persue your immigrants visa application at NVC with in 1 year the case will be terminated

Yes, the national visa center holds the case for processing, and the immigrant has to file the non-immigrant or immigrant visa application, pay the fees, and upload documents if necessary to the CEAC website. Once the file is ready to be sent to the US consulate overseas, the immigrant must schedule an interview, and also make an appointment for a medical examination. You should retain counsel to handle the case from start to finish. Some of us charge very affordable flat fee for marriage based green card cases and fiancé based visa cases.