People can then change it if they'd prefer something else.
When subscribers set up a filter to put the list mail in its own mailbox they can remove the subject tag, as it takes up space in the subject field. Whichever you have as your default, you should explain in your Welcome message how they can change it to the other. The Dual Header Option. There are various other header options. This is only needed on some corporate LANs that go through a gateway.