This is the first heading button, the one with the blue line all the way down the icon. Since it is the first heading, it is a level one heading.

If you click on it again, you get another heading on the same level.

This moves down on level.

To create another heading on the same level, use the button. This is different from MS Office where you would click on the lvl 2 button.

The level down button moves one level down again. This time to level 3. This Wiki is set up for three levels.