Hi David,
It differs in the way it is handled inside ArduPilot. The gimbal module sends a command that is executed immediately without the need to save it. The flight plan is saved to be used later when you are running a mission in Auto mode - there might be a bug in that, there’s a report here in the forum: APM:Copter V3.4.2-rc2,the command DO_DIGICAM_CONTROL does not trigger the servo/camera