The calculator appears when you click a numeric field. Use it to calculate integer values or timecode. The timecode calculator appears only when you click a field that accepts timecode values and when you display preferences that are set to use timecode, and not frame numbers.
By default, the calculator appears over the field you clicked to open it. You can move it to another location by dragging the grey bar along its upper edge. You can also set calculator preferences to open the calculator at other locations. See User Interface Preferences.
There is another type of calculator, the keycode calculator, used for inputting keycode. The keycode calculator appears when you click a Keycode field. See About the Keycode Calculator.