Ranges of insulation resistance tester U1451A

Hello,

I'm interested in an insulation resistance tester U1451A. But it is not clear for me, what are the measurement ranges. I assumed that the ranges are

6MΩ

60MΩ

600MΩ

6GΩ

60GΩ

but this is not possible, because the meter is advertised with "up to 66GΩ", and not 60GΩ.

In the manual is written (table 5-7):

6MΩ

60MΩ

<500MΩ

~200GΩ

This is very unclear for me. What means "<500MΩ". Why not 500MΩ or 600MΩ?

Is there no range between 500MΩ and 200GΩ?

Why 200GΩ? The meter is only specified up to 66GΩ.

Can somebody help me?

I need the values for 500V testing voltage.

Best regards

akapuma