In the user interface, the expression "Starting Level" implies that this is the level of the menu which is to display. E.g. if I want to display the 2nd level links, I set it to "2nd level". But if I set the starting level to 2, nothing displays. Instad, I have to set it to level 1 and adjust with "Fixed parent item".
That's confusing. I don't understand what Starting level is for if not the level of the menu to display. ("Blocks that start with the 2nd level or deeper will only be visible when the trail to the active menu item is in the block’s tree." -> what is the block's tree?)