Text from BMW docs:
Self-leveling rear air suspension(optional 3.0i, standard 4.4i &
4.8is)Air springs replace conventional coil springs in the rear suspension.
To keep the X5 level even when heavy loads are carried
(cargo area or trailer tongue load), ride-height sensors recognize
changes in the vehicle’s attitude and signal an electrically powered
air compressor to increase or decrease the pressure in the
air springs.

Adjustable ride height (optional 3.0i & 4.4i, requires
Sport Package; standard 4.8is)
The 3.0i option includes the self-leveling rear suspension and
adjustable ride height; as selfleveling rear air suspension is
standard on the 4.4i, the option for that model is for adjustable
ride height only and is therefore priced lower. The full system
with adjustable ride height is standard on the 4.8is.
With this system, air springs are the springing medium at all four
wheels. Their self-leveling effect at the rear is as described under
self-leveling rear air suspension, above.
Adjustable ride height allows the driver to select any of three
ride heights for the entire vehicle. Via a dash switch to the left of
the steering column, the height can be set to –
• Normal
• Offroad (25 mm/1.0 in. higher
than normal) for increased
ground clearance
• Access (35 mm/1.4 in. lower
than normal) for greater ease
of entry and exit.

The offroad setting can be used at speeds up to approximately
31 mph, at which speed the vehicle automatically returns to
the normal setting if offroad has been selected. The access
setting remains effective until the vehicle reaches approximately
22 mph, or can be engaged if the vehicle is traveling approximately
16 mph or less. All adjustments occur steplessly, and the pneumatics are controlled to effect an essentially even raising or lowering at all
four wheels – although under some conditions of loading one
end of the vehicle may move slightly more quickly than the
other. If the front or rear moves more quickly, the control system
will stop motion there until the other end catches up. Three LEDs indicate the system’s current state:
A steadily illuminating LED indicates that a particular level has been reached; a blinking LED indicates that a level has
been selected and the system is adjusting.
If height adjustment is attempted while a trailer is attached to the
vehicle, the following occurs:
• If already at normal height, the vehicle cannot be raised or lowered.
• If at one of the other settings and the vehicle is at a standstill,
the driver can return the vehicle to normal height, after which it cannot be raised or lowered.
• If the vehicle begins moving at one of the other settings, it will
automatically return to the normal height at 22 mph (if in access) or 31 mph (if in offroad).

X5 4.8is features
4.8-liter Valvetronic V-8 engine
Whereas the 4.4i’s engine is the new 4.4-liter Valvetronic V-8, the
new 4.8is model gets an even larger, more powerful version of
this advanced engine concept.

Specially calibrated 6-speed automatic transmission
Like the 4.4i model, the 4.8is comes standard with a 6-speed
automatic transmission; its calibration, however, is sportier
and specific to this model.

Standard adjustable ride height and self-leveling
This is the only X5 model on which both the adjustable ride
height and self-leveling rear suspension (via air springs front
and rear) are standard. The 4.8is’ spring and shock-absorber
calibration is also firmer, thus sportier, than that of even a
Sport Package-equipped 4.4i model.