Used only a few times. This is an accessory specifically for Sigma's Contemporary, Art, and Sports line lenses, and it's used by connecting to a PC via a USB cable. The main functions are as follows: Lens firmware updates Adjustment of focus position For lenses equipped with a custom mode switch, you can also select AF speed, adjust the focus limiter, and adjust the OS (optical stabilization). These adjustments are made using the dedicated software "SIGMA Optimization Pro". Sigma SIGMA NIKON For Nikon