IV Rank: where current IV sits in the 52-week high-to-low range, expressed 0–100. Rank 80 means current IV is 80% of the way from the year's low to the year's high. IV Percentile: the percent of trading days in the past 52 weeks where IV closed below the current level. Percentile 80 means current IV is higher than 80% of the past year's daily closes. Both are useful; they answer slightly different questions.
Practical heuristics: IV rank above 50 is the classic premium-selling threshold. Above 70 is high-IV territory where premium structures (condors, strangles, credit spreads) get the math heavily in their favor. Below 30 is low-IV territory where premium-buying structures (long calls, long puts, calendars) become more attractive. Outside the 30–70 band is where the easiest decisions live.