Once you have configured the custom relationship permissions, you will have the ability to give this custom relationship visibility and editing into the growth plan(s) of the employee(s) they have been set to oversee.
To access this, the person who has been given this permission can go into their "Your team" tab. From there, they'll click the ellipsis of the user they have been set to oversee and select "View growth plan".
From here, they will be able to view, edit, or create growth areas within the growth plan.
This is also accessible by navigating to the user's profile and selecting, "View growth plan."