Difficulty is measure of how ambitious a Key Result is and there are 3 levels
Moonshot
A nearly impossible goal that will inspire out of the box thinking. Even partial achievement of Moonshots is often a huge win.
Aspirational
A significant stretch but it’s not impossible.
Must Hit 100%
It’s a goal that achievable and should hit 100%.
Difficulty is displayed on every Key Result and the difficulty of an Objective is calculated as the weighted average of the difficulty of its Key Results
The Difficulty is set when creating a Key Result and can be changed at anytime using the menu on the Key Result display.
The difficulty label and values can be changed - see How to change Success Level & Difficulty values