Sub Objective and Hierarchy:
This feature lets you create a sub-objective for your objective and create a hierarchical order for your OKRs.
How it works:
Step 1- After creating Top level objective, just select the expand option
Step 2- There you will see an option to Add new objective.
Step 3 - Just select the option and create new objective.
P.S. You can create sub objective for every objective by just selecting expand option and creating new objectives.
This feature serves the great purpose of to provide a tree view for your goals as per your organizational structure or your requirements.
Having all your sub objectives under your one ultimate goal helps one to focus on the bigger goals as well as micro goals to achieve it.