The following rules apply to all constructed decks:
The Standard format is a fresh and exciting way to play HEX constructed, with new cards and strategies taking center stage often. In this format, you construct a deck containing a minimum of sixty cards using cards and gems from the latest sets. When a new card set is released, the list of Standard legal cards or gems will rotate. (Set legality will update with each odd numbered set, while gems will be updated on even numbered sets.) When a set rotates out of Standard legality, the associated booster packs will no longer be featured in the HEX store for purchase.
The Immortal format is a place where powerful cards and tactics from throughout HEX’s history can continue to compete for glory. The Immortal format allows you to play with any PvP cards, gems, or champions, regardless of set. In this format, players construct decks containing a minimum of sixty cards from across any PvP set. The only cards excluded in this format are cards on the Immortal banned list, or PvE cards.
The cards, gems, and champions in this list are legal in this format, but are being scrutinized and evaluated due to balance or gameplay concerns. Items in this list have a much higher likelihood of being added to the banned list in the future than other existing cards.
Developer explanation here.
In these PvE formats, all cards are legal unless they appear on the banned list. Only the Standard legal gems are available when constructing a PvE deck. Also, keep in mind that you may still be limited by your character’s shard alignment grid or mercenary deck building rules while in Campaign mode.