What is the "Cluster tip area" ?


When erasing unused space I have the choice to erase the "Cluster tip area" as well. What is the "Cluster tip area" ?
From the Eraser Help file:

Cluster tip

The unused area at the end of the last cluster allocated by a file is called the cluster tip (or the slack space). This unused area is present in most files because space can be allocated only as cluster sized blocks and the contents of the file rarely completely fill all allocated clusters.