Thanks for all comments. I just received the meter.
One of the attached pictures shows that the resolution at 500V is 10k. This is not reflected in the manual where under 500V, the measuring range is listed as 1MΩ to 20GΩ. I guess this 1MΩ is not meant for the resolution.
10K or not, I am not sure about the accuracy, though the number displayed is stable, repeatable and consistent with the trend:
6.1 MΩ at 1000V
6.65MΩ at 500V
6.9 MΩ at 250V
7.36MΩ at DMM
The other picture shows 19.12GΩ insulation resistance for a thin USB cable at 2500V. I also tested a few other wall power cables, all showed "OL", so I end up testing this USB cable.
Ok, just find out. It does not measure resistors below 1M. The meter showed all zeros and beeped constantly. At 2.2M, the meter showed 2.18M, then also beeped constantly.