This symbol shows that the entry has been marked for use by the Retrieve entry, Throw entry(ies) and Jump to mark commands. Only one entry can be marked at a time. And the overall model Title cannot be marked because it cannot be moved.
If a list ends with a series of blank entries, they will automatically be removed, when moving to another part of the model. If one of these blank entries is Marked, then this will become the last entry in the list.
See Mark on/off for usage details.