With the internal storage space of the PC-FX being limited to 32 kB, you’re bound to need more storage after playing just a few games. You could source an expensive original FX-BMP but then you’re dealing with batteries and lost game saves.
The FX-nvBMP is a replacement “Backup Memory Pack” for the PC-FX. The “nv” stands for “non-volatile”, which means your game saves won’t be lost if you run out of power. This version of the FX-nvBMP is based on David Shadof’s original design.