In the image below, why has the mouse pointer changed to a black plus sign?

The image shows an Excel worksheet with the number $2$ entered into cell A$1,$ the number $4$ entered into cell A$2$ and the number $6$ entered into cell A$3.$ Cell A$1$ is activated with the mouse pointer hovered over the lower right corner of the cell.

Responses

Cell A$1$ can now be clicked and dragged to a new location on the worksheet.

Cell A$1$ can now be clicked and dragged to a new location on the worksheet.

The data in cell A$1$ begins a logical sequence.

The data in cell A$1$ begins a logical sequence.

The mouse can now be doubled clicked to copy the data in cell A$1.$

The mouse can now be doubled clicked to copy the data in cell A$1.$

The mouse pointer has been placed over the Fill Handle.