>> We ensure that only one copy of every record is read from disk and delivered over the network by including the copy set in the header of every record copy. A simple server-side filtering scheme based on copy sets coupled with a dense copy set index guarantees that in steady state only one node in the copy set would read and delivery a copy of the record to a particular reader.

Can someone explain/expand on the above please? I've read the article a couple of times and tried to understand the above paragraph in context but I don't get it.