1st? 2nd? What is the criterion to distinguish the order of NFTs?

1st NFT is an NFT that has been sold since December of last year.
– Tyrion (Combatant male)
– Beyonce (Lumberjack female)
– Hoober (Miner male)
– Angelica (Farmer female)

2nd NFT refers to new heroes that will be revealed starting March 28th.
– Glados (Miner female)
– Klose (Lumberjack male)
– Olivia (Combatant female)
– Dave (Farmer male)