The Nucleus Menu displays your progress towards the various Monster collections on Magical Nexus. Collections are sorted by Rarity and Number of Elements; tap on a specific Number-Rhombus to view that collection in the island's Book of Monsters. Once you've Transposed all of the Monsters in a given Number collection to Magical Nexus, the corresponding Rarity symbol will become more detailed and one of the Structure's Rhombi will light up.