The ADDER RMK12-BP is an optional blanking plate for the RMK-12 rack mount for the ALIF4000 series, like the ADR-ALIF4001R-US. In order to maintain suitable cooling airflow, if a single module is installed within the rack mount it's recommended to install an optional blanking plate like the RMK12-BP.
Note: The outer edge of the blanking plate needs to overlap the ears of the rack mount itself, so you need to either install the blanking plate before the tray is installed or briefly remove the mounting bolts on the blanked side (while supporting the weight of the tray) to allow the blanking plate to be put in place.