Since Season 13, any affix whose tooltip literally contains the word 'Multiplier' (e.g. 'x20% Critical Strike Damage Multiplier') sums with other same-worded sources into ONE combined Summed Multiplier bucket for that category, rather than each being its own independent x% factor.
This is a genuinely confusable rule: two uniques both boosting 'Critical Strike Damage' can be in completely different buckets depending on exact wording. Plain '[+]Critical Strike Damage%' (no 'Multiplier' word) goes into the general Additive Damage Multiplier bucket alongside everything else. 'x[N]% Critical Strike Damage Multiplier' (with the word 'Multiplier') instead sums with other Summed-Multiplier sources of the same category (Critical Strike Damage, Damage over Time, Vulnerable Damage, or a specific/'All' elemental type) into one shared bucket, which then applies as a single Global Multiplier alongside - not instead of - the other x% buckets.
This is a deliberate Season 13 design choice to stop certain categories (especially Crit Damage and DoT) from multiplying together uncontrollably when several 'Multiplier'-worded uniques were combined; grouping them into one additive-within-category sum, then applying that sum once, tames the scaling.
Two hypothetical 'x30% Critical Strike Damage Multiplier' sources do NOT give (1.30*1.30)=1.69x; they give (1+0.30+0.30)=1.60x, applied once as one Global Multiplier bucket alongside your other independent x% sources.
Read tooltips literally for the word 'Multiplier.' If you already own one 'x% ... Multiplier' source in a category, a second one of the same category is worth much less than its face value implies; a differently-worded plain 'x%' or '+%' source of the same nominal category is not affected and may be the better pickup.
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