Data Transfers · Setting up a transfer
What does “Use … root” mean in the cloud picker?
Short answer: It uses the whole account or device — its top level — as the source or destination. As a destination, prefer a dedicated folder instead, especially for a one-way sync, which removes anything not in the source.
The root is the very top level of an account or device — everything in it. Choosing Use … root from the cloud picker fills the field with the whole account without opening the folder browser.
As a source, that means the transfer covers the entire account. As a destination, files land at the top level, mixed in with whatever is already there — usually you'll want to Browse and pick or create a dedicated folder instead, especially for a one-way sync, which removes anything at the destination that isn't in the source.
Related: Pick a folder inside a cloud account · Does Sync (one-way) delete files?