A set is private when you create it. You choose who else can have it in the set’s access dialog, and the three levels are exclusive — a set is at one of them at a time.

To me only. Nobody else can open it.

To anyone with the link. The set stays private, but whoever has the link can open and read it. The link is read-only — shared editing is announced in the dialog but is not available yet. Refreshing the link replaces it, and the previous one stops working.

To everyone. The set becomes readable by anyone, signed in or not, and it is added to the search page so people can find it without a link. Making it private again removes it from search; it is not deleted, and people who already added it keep their copy.

Two things that surprise teachers:

Assigning a task does not give the student the set. The student sees the words while they do the task, and that is all: the set itself is not opened for them and does not appear among their sets. If you want a student to have the set, share it.

Adding someone else’s set adds a reference, not a copy. Removing it later removes only your own access — the set stays with its owner. But your tasks built on that set go with it, so remove a shared set only when you no longer need the tasks you made from it.