With AC coupling, the trigger level will vary with duty cycle so having a numeric value for the trigger level may be misleading.
That's a good explanation - thanks.
I'm still not sure why there would be a difference between handling trigger cursor display between HF and LF rejection. I'm not even sure if they are AC or DC coupled.
HF reject means a low pass filter is inserted into the trigger path so it is DC coupled. LF reject means that a high pass filter is inserted into the trigger path so it is AC coupled. Neither will allow an absolute trigger level measurement but for different reasons.
A Tektronix 2247A is a good real world example. AC or DC coupling of the vertical input does not affect the trigger level display because the oscilloscope measures the levels after that point and the trigger level just has to match what is on the CRT but AC, LF reject, and HF reject trigger coupling disables the trigger level display because either the DC information is lost or in the case of HF reject coupling, the loss of high frequencies causes the trigger to be late.
A Tektronix 2232 on the other hand always displays the trigger level if a vertical input is the trigger source but in LF reject trigger coupling, the trigger level is only accurate in a relative sense and does not represent the absolute trigger level as shown on the CRT. In HF reject mode, the extra delay caused by loss of the high frequencies causes a similar error.