From what I've seen, in context, a whataboutism is concerned about the argument of moral authority and precedent from the person accusing a AES or progressive of something bad...
Here's some rules:
Apply it when it relates to the precedent of incidents that seem exceptionally bad, when committed by Actually-Existing-Socialist countries, but are ignored, if not justified in Capitalist ones... they could be explained as historical thinking of the time or justified under circumstances
For example: https://hexbear.net/post/2329752 (Homosexuality laws compared between historical Soviet Union and the West, lemmygrad meme)
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However, don't use irrelevant ones to detract from past mistakes, they make you look daft and aloof
I'll explain more later on...
whataboutism
Fallacy fallacy