Hi Michael,
The active clamp will clamp the Vout to +/-0.7V. When there is a loading resistor at the output, Vout will then settle to 0V with a RC time constant. On fig 6 & 7 (page 9) of the datasheet, Rload is 100 Ohm & Cload is 330pF.
GNDHI is HVGND.
Active clamp is similar to the output stage.
Regards,
Charles