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...
it's not about the lib you're talking to directly, it's about the ones reading the conversation who are less on the defense against your points. Literally right now I don't expect to change your mind on the topic but rather to encourage and reassure other people who will see this.