When does a small plus arrow appear during an Excel operation?

Answer 1

When you are copying something. If you use the fill handle, by putting the cursor over the bottom right corner of the active cell, you will get a solid black cross. You can then drag down to fill or copy. What you are particularly referring to is when you put the cursor on the edge of a cell and press and hold the Ctrl key, you will see a small cross above the arrow. You can then drag and copy the cell or cells you have selected, and even other things like graphics that have been drawn on the screen.