The Select All button sits at the upper left of all worksheets, at the origin of row and column labels. You can use the Select All button to quickly select all cells in a worksheet.
When entering a formula, you can can click the Select All button to enter a reference an all cells in a worksheet. For example, if you create a formula based on the COUNTA function, and click Select All in Sheet2 when entering the first argument, you'll get:
The reference Sheet2!1:1048576 is a range that includes every row in Sheet2, automatically entered by Excel when you click Select All.