LT trim
ST trim
Long term and short term trims are corrections the ECU makes for fueling errors it detects during closed loop fueling mode. This is at light load / part throttle operation and usually limited to lower RPM's. The open loop fueling mode (WOT, high RPM high load) then uses thes corrections but does not actually determine if they are delivering the right amount of fuel. Open loop specifically uses the long term trim. SOOO if you have a recent mod, and the long term trim has not corrected for it yet, you will see no correction at WOT. Hence all the lean engine blowing up. Edit: As randy said, PE negates the fueling correction.
Narrowband
Wideband
Wideband 02's detect (over there voltage range) a complete air fuel ratio spectrum. "narrowband" or Switching 02 sensors only detect whether the air fuel ratio is greater or less than stoichiometric (14.7:1). The specific amount over or under is not known, so the ECU corrects slowly, usually arriving at a correction within the 20% maximum limitation. IF it does not arrive at a correction within the 20%, the Check engine light is turned on.
When does it transition to PE mode, wide open throttle only? Is PE mode control a function of how the ECM is programmed (open loop)?
The ECU is programmed for a lot of different fueling applications, which are assigned based on priority, simplest form being whichever one is richest, and the ECU chooses an air fuel ratio from that. PE is activated typically at high Load / Higher RPM situations. This table is often used by tuners (logic of which is debateable) to adjust WOT fueling for mods.
I've heard the ECM's "learn" and adjust to actual operating conditions - octane, small mods, etc. What is the limitation of how much the ECM can compensate???
Yes there is a limit, but we don't know what it is. You can trick it into all kinds of stuff... pressure regulators on boost solenoids, fake 02 sensors, etc. etc. etc. But that really has nothing to do with changing the calibration and aren't very robust solutions.