What is marking an item as "Landmark" in the Eclipse IDE ("Mark as Landmark")?

Question

I’m studying the Mylyn of Eclipse IDE and came across the option "Mark as Landmark" which appears available for some items in the Package Explorer, and I got a few questions about what it’s for, what it’s for, and how to use:

In the gif above, I have an Active Task, and, the Package Explorer is with the option "Focus on Active Task" linked, from this I do the following actions:

1. Click on the File Main.java right-click and check that option "Mark as Landmark" is not available to him; because it is not available?
2. Click on the button [+] which is to the right of the Archive Main.java to show his "children" who are not part of the Context, and then comes the class Main () that is inside the Archive;
3. Then I select the class Main that appeared, and right-click on it, see that for it there is the option "Mark as Landmark";
4. Click on [+] class Main and her "children" who are not part of the Context, which are: the attribute UI (), the method main(String[]) () and the method start(Stage) ();
5. For each of the children of Main which were opened in the previous action, they have the option "Mark as Landmark" available, but in gif I show this only for the method main(String[]), by right-clicking on it;
6. I click on the option "Mark as Landmark" causing the method main(String[]) is marked, note that it was in bold;
7. I right-click on the method again main(String[]), looking for an option that allows you to uncheck it as "Landmark", but I can’t find it! and the option "Mark as Landmark" is still available to him even though he is already marked (because? will be a bug?). How can I Deselect the item as Landmark?

Main Doubts:

• The option "Mark as Landmark" seems to be available only for Items that are part of the Context of the Active Task, so that I think it is an option relative to the Contexts of Tasks of Mylyn; that’s right?
• What is and/or what the option is for "Mark as Landmark"?
• In which cases it is interesting to use the option "Mark as Landmark"?
• How to use the option correctly "Mark as Landmark"?