What I did:
Created a new project in Other Projects. Moved it to a Category called the Grid. Wanted to add the new project as a sub project if you like of Battery Storage
Could not do it. Not unexpected. As the Category Sub Category Projects system is suboptimal. And use of Sub Categories is discouraged by design.
What I expected:
To simply drop the new project as a subproject.
Work round, created four additional projects and transferred notes appropriately. Time consuming and inefficient, though the category is now better organized.
Things that might be helpful to know (Agenda version, OS and model, etc):