The BD is an overcurrent trip switch, 1A1B. For field installation order S29450 and S29451 (for H and J frames), or order S29450 and put it in the SDE slot (L, M, P, and R frames).
See link for more details:https://www.se.com/us/en/search/S29450
Auxiliary or Alarm Switch for PowerPact Molded Case Circuit Breakers CB OVERCURRENT TRIP SWITCH ADAPTER
CTA-ID : 2054844