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.

That's not the same as your contention thatOf course you can't expand the selection to the left of the first selected cell when using the mouse (unless you hold down the Shift key), but neither do you need to select all the cells to the right of the selected cells if you don't want to number all columns to the right. In that case, don't select them - you can simply drag the cursor down. If you want to number cells to the left of the cell that's numbered on the first row, simply make another selection after doing the first block and repeat the process. The numbering sequence will update to reflect the newly-numbered cells.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]