Product code. According to relevant information query, the NR in the Panasonic refrigerator model NR-D521CX-7 represents the product code of the Panasonic refrigerator, and D521 represents the volume of this refrigerator, which is 521 liters. C means that this is an ordinary refrigerator, while X means that this refrigerator uses a very energy-saving compressor. The final number 7 may refer to a specific region or market for this refrigerator, or may represent some minor improvements or updated versions.