The altimeter shows the aircraft’s altitude above sea-level.
The input channel to the Altimeter widget is the height in feet.
To assign channels to widget please take a look at the page Channel selector.
If the indication of the height is under 10000 ft, the area with black and white lines appears in the field below.
The image below show the height of approximatelly 1970 ft.
The longest and thinest pointer shows the height of 10000 ft, the moddle point the height in 100 ft and the shortest pointer the height is 1000 ft.
If the indication of height is above 10000 ft, the field with black and white stripes is hidden.