You start with one random writing key; to let someone write to a folder, that folder gets its own key pair, pointer and bag; the guest can change only that subtree, under a quota if you like.

It is how write access is delegated without sharing the root key.

You start with one random writing key pair. It controls writes to your whole file system.

To let a friend write to one folder, you do not share that key. You cut that folder off, onto its own new key pair.

That key gets its own pointer and its own bag of blobs. A writing subspace.

Your friend holds the subspace key. They can change that folder. Nothing above it.

A subspace can carry a quota. A guest cannot fill your whole store.

Check yourself. What does a friend with a subspace key control?