How should I read the voltage ratings on this terminal strip?
What you have there is a bad form of technical documentation - too terse to be easily comprehensible. So what I did was to go to the manufacturer's site (Wago) and looked at the tabular form of specifications in their catalog (search for 236-102). Another helpful clue is the vertical line separating two voltage ratings which matches the split between metric and AWG. It turned out the split is also for two separate ratings depending on the certifying authority. So here's what those numbers would mean:

On left side: [Rated Voltage]/[Rated Surge Voltage]/[Applicable Environment/Pollution Category], [Nominal Current]

These ratings are based on IEC/EN 60664-1. As the standard is only available for a fee, here's a handy guide on Pollution Categories. Also, here's Wago's detail sheet on test methods and parameters.

On right side: [Rated Voltage], [Nominal Current] [Certification]

Thus for CSA, the certified ratings are 300V and 15A.