Remember all altitudes are not created equal.
- Pressure altitude is altitude above a set reference datum
- Density altitude is pressure altitude corrected for nonstandard temperature.
- Absolute Altitude is the actual altitude above the ground at your present location above the planet
- Indicated Altitude is your altitude above Mean Sea Level (MSL) corrected for pressure variations at that location.
Flight levels are a pressure altitude reading, that is a height above a standard datum (MSL at an ambient pressure of 29.92" Hg.). This is used for a standard height reference at high altitudes (>18,000 ft MSL) so crews operating there can make use of a universal reference datum. An airplane cruising at FL390 may or may not actually be 39,000 ft above MSL depending on ambient conditions but maintains a standard height from a universal datum for traffic separation in this airspace.
Lower altitudes will make use of indicated altimeter readings to indicate height above MSL. This is favored because of the aircraft's proximity to terrain, providing appropriate reading for obstacle clearance, etc.