What does "1000 VRMS CAT II, 2500 Vpk" on a voltage probe label mean?
This category number refers to the Insulation Standard for location of a power source for human safety.   Arc flash energy ( and thus risk to thermal explosions) increases in each zone by increasing category numbers is implied by the unstated current and energy available, if an arc were to occur, not the actual breakdown voltage rating.

So a Cat II probe is only suitable for Cat I &amp; II zones.