When throwing splash or lingering potions, they'll create white particles, either white potion particles or snow particles.
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Can confirm in 20w51a, they mean that white particles appear when a splash potion or lingering potion is splashed. Also, as you can see in the screenshot that I just attached, when I splashed down a lingering potion of poison II, there were white particles.
Duplicates MC-91049
Please provide a screenshot of this occurring with your F3 debug screen enabled.