Well I must say since I never use OBS, I am not 100% familiar with it's operation. I just looked at page 12 in the manual (version I got when I purchased) and below is straight from the manual. . . .
OBSTACLE AVOIDANCE Switch
With sustainable GPS locked, the RealSense can be activated ONLY in angle mode by being switched
on except for the condition that the copter descends vertically at low speed with landing gear down.
When the RealSense is activated, the front motor LEDs will blink white and the ST16S screen will display
a [symbol]. When the OBSTACLE AVOID is switched on, the screen will pop up ‘OBS activated’ .
IMPORTANT NOTE: The environment requirement such as flight track might change depending on the
situation. Only when the icon on the ST16S screen is solid green, it means the OBSTACLE AVOID function
is activated.
So reading between the lines of the above, I am seeing what
@claudius62 is saying. Though Yuneec never differentiates or tells us the steps to 'activate' RealSense and OBS, it does look like they are saying this is two different things thus; position two turns on the RS and the Display on the screen and thus shows you what it is seeing but it does not use this info.
Only when OBS is activated (third position)does it use the info from the RealSense to actively intervene in the flight.