If you have a naturally aspirated engine, you have an "ADA". This device will regulate the fuel based upon ambient pressure. The higher you go, the lower the pressure, the less the fuel provided.

The "ALDA" does the same thing as the "ADA", however, it also provides for additional fuel for turbocharged vehicles when they get into boost from the turbo. The turbo causes a climb in the manifold pressure and the ALDA advances the rack to provide additional fuel to take advantage of the additional air.