TumbledMTG is a 'rotating' format. There is a large master pool, and each season a legal card list is generated from that master set. Every card in the master set is 'rolled' for, with the following odds of being legal for any given season:
Mythic Rare -%9
Some cards are 'bundled' together and always rotate in as a group. If a card is part of a bundle (like many dual lands are) it is indicated above the artist credit.