The codes tell which sensor is reading the information. It's not an elaborate system- each sensor that can cause a code will either report higher or lower voltage than normal. The knock sensor either hears knock or nothing- either will cause a code to be stored. The IC (Ignition Control) Module will also cause codes to be stored. The codes have been posted here several times and they're the same ones GM has used for decades.
To read the codes on the MIL (Malfunction Indicator Lamp), you need to connect terminals A and B on the DLC (Data Link Connector), which is sometimes called the ALDL (Assembly Line Data Link). If you don't use the MIL, you can use a test light on two of the other DLC terminals. You won't see actual voltages or percentages with a scan tool.
I wouldn't recommend changing any of the values in the calibration program, if the software allows it. Changing the tuning can result in catastrophic failure if you try to guess.