Elios 2 has several flight modes which can be selected by the pilot. Each have their own properties and uses.
ASSIST is the default control mode, and must be engaged at the beginning of a flight. In this mode, several optical sensors around the drone will measure its distance and motion with respect to nearby objects. It uses this information to automatically remain perfectly stationary.
However, optical sensors have limited range, and the quality of their measurements can be reduced by dust, direct sunlight, or featureless surfaces.
In the ATTI flight mode, the pilot's inputs directly control the attitude of the drone and the optical sensors are not used. This control mode must be selected if the pilot notices erratic flight behavior due to the limitations of the optical sensors.
Some flight environments are more demanding on the stability system (and pilot) than others. You can use the difficulty assessment sheet to estimate the difficulty level in function of several environmental factors.