¿What's the use of "public" button?

[Evoplure]

First of all, excuse me if this cuestion is obvious, but I've got a little mess with it.

I try to use the Visible and Public buttons with articles, and I find the following combinations possible:

1.

2.

3.

4.

I can understand what is the difference between 1 and the others, but I can't understand the difference between 2, 3 and 4.

¿And why can these options be changed by an user who is not the owner of the article?

Thank you very much

Greetings

[pico]

Hi

visible is clear I think - Visible or not
public is for the comming User/Group-Right-Management - so if a Page is Public everybody can see it - if it is not public only 'Logged In'-Users can see it

OK?