Any card whose collector number is higher than the set's printed total, such as 185/162. Secret Rares, Illustration Rares and other full arts are counted above the base set.
For new players
30th Celebration prints its numbers out of 128 (Mew ex is 152/128), but the official checklist runs to 158: cards 129 to 158 are the Illustration Rares, Special Illustration Rares and the two Futuristic rares.