What does SD mean in Xinbao reducer VRSFS-40C-750-SD?

The general naming rule is series number + mounting form + reducer specification number + motor seat number + motor specification code + nail core length code + motor pole number + brake type + other additional technical parameters

BM with brake

HF manual release device

SF thermal protection

D is direct connection

Truly confused you can contact the manufacturer, they are obliged to explain to the customer.