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As Naxxramas requires you to only kill through one wing before being able to take on the end bosses of all wings and progress to Frostwyrm Lair the progression table is somewhat more complicated.
For example if you've had cleared through the Abomination Wing you would have Completed for that wing. If you then continued on and killed Maexxna from the Spider wing without clearing the remainder of the wing's bosses you would place Maexxna in the column, meaning you are attempting that boss.