Explanation
The pickup is a strip (electret film) that sits right under the saddle, which is applying the pressure produced by the strings.
AST = acoustic soundboard transducer
The pickup is a electret film that is sticked underneath the bridge inside of the guitar (under the soundboard). The difference is, that it picks up the signal or "sound" of the entire guitar-top rather than the strings only.
The AST is the new alternative for the condenser mic.