4♥ rebid is explained as "5+H; 21- HCP; 19-22 total points"
4♦ rebid is explained as "5+H; 11-21 HCP; biddable D; 12-22 total points"
3♦ rebid is explained as "3+D; 5+H; 2-11 8421HCP in D; 18 total points"
In terms of total points (not necessarily HCP), 4♦ should be at least as strong as 4♥, since it forces game and invites slam, as opposed to simply being game.
3♦ is exactly 18, not 17 and not 19? Responder is 6-10.

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