Here is a link to an image I uploaded showing what happens when I left click and drag the cursor to gather the group of cells I would like to number.
http://i.imgur.com/WjEaUXk.png

The cursor will not go to the left of the starting point, which I marked with an "X", it only selects anything after and directly below. Meaning, I could not start in column 5 and end with say, the last cell in column 6, because it would select the entire column 6 column. To put it simply, it will only select in complete blocks (of cells), and only to the direct right of the starting point.