Com - "common" port that can be switched to pass air from NO or NC
NO - normally open port. When the solenoid is OFF, air passes from Com to NO (or the other way - it's not a diode ;) )
NC - normally closed port. When the solenoid is ON, air passes from Com to NC
Com is connected to the boost source (inlet manifold, but rather pre-throttle. The closer to compressor the safer)
NO is connected to the actuator (wastegate-bottom, so it gets boost with an unpowered solenoid)
NC is connected to fresh air (in a MAP/AlphaN car. MAF is slightly different...)
If you use a (1mm or so) restriction to feed boost to wastegate-bottom and control by "bleeding" airpressure, than the NC port is used (which bleeds to Com when the solenoid is powered). Bleeding from Com would not work well, as it bleeds to NO without power, and to NC with power (blind-plugging NO could work, but just an unnecessary point of uncertainty).
Yet to be re-edited
You then have a duty cycle table (TPS against RPM) and this is used to stop the actuator "seeing" pressure. When you want to build boost fast, the duty cycle approaches 100%.
The "sense" of this changes slightly for an external wastegate as these have a 2 port actuator - wastegate-bottom for "open" and wastegate-top for "hold closed". I will throw some diagrams together if you like...
More info about boostcontrol, even plumbing diagrams, look at the the following page: BoostController - DB