Active working sets in Eclipse

I am familiar with Eclipse and with working sets. But there is something I don't understand: Why are there new menu options (with some working set names) just under "Select Working Set...", "Deselect Working Set", "Edit Active Working Set.." options?

Clicking on one of these working set names, a dot appears on the left and the project explorer view contains just this working set.

It seems this is the active working set. Is that right?

But, I don't understand how I can force a working set to be in the menu options so I can select it to be active. In my workspace, I have 10 working sets but just 2 of them are in the menu options, I don't know why.

The list of Working Set that you see below "Select Working Set...", "Deselect Working Set", "Edit Active Working Set.." options are the ones which were selected recently.

When clicking on one of these working set names, a dot appears on the left and the project explorer view contains just this working set. It seems this is the active working set. Is that right ?

Yes, the one with dot is the active working set.

But, I don't understand how I can force a working set to be in the menu options so I can select it to be active. In my workspace, I have 10 working sets but just 2 of them are in the menu options, I don't know why.

If you want to have a specific working set (say working set Last) to appear in this menu then click on Select Working Set... and in the dialog which appear, just select that particular working set and click OK. As shown in the diagram: