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It's here in the advanced settings right before you publish a post. It merely says that a percentage of your post rewards will be sent to @steem.dao or another account which curators can then see when they mouse over they click on the post payout button here:

so it can affect your curation (depending on content/message/etc) but doesn't penalize curation rewards the way declined payouts would.

Oh okay, very interesting thanks!
So does that just help to curate other people's content then?