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Each player chooses from the lands they control a land of each basic land type, then sacrifices the rest.
You sacrifice all lands other than the chosen ones. Since you can only choose lands with basic land types, any non-basic lands that don’t have a basic land type (even if they have some other land type, as in the case of Cloudpost) are necessarily going to be sacrificed.
If a land counts as multiple basic land types, you can choose it for either or for both of those land types.
