If crossposting is allowed, this setting determines what people who are members of more than one list will receive. An example will help clarify how this feature works. Say you have several announcement lists, where you post articles about various topics.
People join the various lists that they are interested in.
Some people join just one list, while others might join several lists. They will receive the message from the first group you sent the message to. This feature is particularly useful with announcement lists, where you want to split your lists into various topics, but often you have messages that apply to several topics. With other list servers, people would receive multiple copies. But with Lyris, the duplicate messages are removed, and people only receive one copy.