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But what happens if you tag @main? Will you end up tagging all instances' main communities?
You can't meaningfully just tag 'main', you need to include an instance too. https://activitypub.academy is good resource to test and verify this stuff out for yourself.
The @asklemmy links to https://berlin.social/@asklemmy@lemmy.world specifically, so I guess you need to specify the instance
I was under the impression that the link interpretation happens on Lemmy's side. The lemmy servers will interpret the mention the best way they can. And I'd suspect that the best way to interpret the @main mention is to prioritize local communities.
I have no clue what happens on mastodon, but on lemmy all interpretation is done on the client.