All potential bonuses

Well I was fairly sure i've seen Mystic Door on a Zakum helm, which is level 50.

Though if that was a Chaos zakum helm your theory holds, but I dont think it was...

If you're talking about jellyflower/lachrymus's helm, it's a chaos zak.

Can someone give me an example illustrating exactly what

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rare secondary

Rare primary, epic secondary

Epic primary, unique secondary

Unique primary

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actually means. To make it simpler, just use % dex on 71-120, so meaning rare = 3%, epic is 6%, and unique is 9%. I'm getting confused exactly about the primary/secondary, and when you list both on one option.