In DAT, append-only logs addressable by a key are called hypercores. When you copy one such log to another computer and write to it, there are suddenly two logs in two different locations. The new location being a fork of the first log. Both logs can be found under the same ID, making it impossible to distinguish them from another.
DAT mitigates this by making every log "single-writer". Only with a secret writeKey you can append data to the log. By convention that secret is