1.These PCBs have an IC which needs 5V to make it work, so the minimum voltage is >5V = 6V, the high voltage rating is determined by the output transistors, which in this case was 90V.
2. The current rating is determined by the current the load is taking and its heating effects on the output transistors. So thhe output transistors can handle X amps, so the module is rated at X amps, you could arrange to have it fan coolled, then its rating could be greater then X
You have to be warned that the published figures might be optimistic i.e. instead of 90V use it at no more then 70V. . .
Frank