What sub-sites are for
Sometimes one location is really made up of several places that each need tracking on their own. A school campus, for example, might have a separate science block, hostel and assembly hall — each getting its own solar installation. Sub-sites let you model this: a parent site holds the group, and each sub-site underneath is a full site in its own right, with its own status, location, cases and documents.
On your lists and dashboards, sub-sites roll up under their parent, so you see both the campus as a whole and each building within it.