While adding or editing your current journals list, you may have seen a link symbol next to one of them. This is to indicate which journal would sync to Day One Classic (version 1.x). Day One Classic supports one journal. Day One 2.0 supports up to ten. Some users have a combination of Day One Classic and Day One 2.0. You can sync between these two versions, but only the journal with the link symbol in Day One 2.0 will sync with Classic.
If you do not have Day One Classic, then it is just for information. Syncing between Day One Classic and Day One 2.0.