Trying to understand "Block formatting context"

In the specification, it states:

“In a block formatting context, boxes are laid out one after the other, vertically”
if we follow the definition of a formatting context, then this example …

<div style="position: absolute;">

        <span>first span</span>

        <span>second span</span>


should layout the spans (boxes) vertically as per the specification. However it still follows the normal flow layout for block and inline elements, so what is exactly the meaning of “Block formatting context” ? and how is it different from the normal flow?