This is used to cover the entire top and left sides - and background. You could ofcourse leave the background transparent, and there by let anything behind it bleed through.
In our example this will be placed in the top right corner of an <h3> element, and therefore it needs to be as high as we expect our <h3> element to be.
This will be used to cover the bottom left corner, and the bottom side. It therefore needs to be as wide as you expect your given "article" to be.
In our example we are placing the bottom right corner in a paragraph. Therefore it needs to be as high as you expect that paragraph to be.
Even though it's hard to see, it's there. This is a 1 pixel high graphic we're repeating along the y-axis of the body text.