It is Altitude Optimization.
It is a straight line of oblique images taken from an outside corner to the middle of the mapped area.
If you are receiving corrections and do not use this and only use nadir images, you will have a vertical shift of your data.
These oblique images help the photogrammetry software "solve" for vertical.
You can also use double grid oblique or the multi-oriented image acquisition schemes to "solve" this vertical shift.
To turn it off, use the Altitude Optimization toggle in the flight planning in DJI Pilot 2, but by doing so will have a vertical shift.
If you do not need or care about vertical then just turn it off.