I’m 99,8% sure it is +/- 600 VDC.
Peaks of sinusoidal 420 VAC RMS should fit nicely in that same range. And i am quite sure that differences between ac and dc measurements are mostly in the code.
What would that lower limit be, other than -600 V? Zero? Something between those should be mentioned in specs.. I started this post with 98% certainty and had to tweak it couple times as i wrote..