The Solid protocol specification or anything similar (it doesn’t have to be that specific protocol).

For example, registering to a website or service actually creates a local secure database/bucket/pod where that website/service organizes/sort/manipulates our data and stores all generated modified data/metadata within our local personnal server, every time we interact with that same external website/service it gets access to the database/bucket previously created. (Ideally) no personnal data should be stored on external servers/machines outside our control and without our explicit consent.