Imagine you have a site diagram like this:
If you are in level C, normally when you click on the “up” navigation element: A – B – C
If you choose not to inherit the top site navigation, the top site will not show up in your navigation element.
For instance, if we say that site C will not use the top site navigation, the “up” element will only show: A – C
Had this at at a customer and took me some time to find it!