Is about the format "Heirloom" on MTGO. The legality of cards in Heirloom is based on a tiered pricing system when the Legal Card List was first made and whenever it is revised with the following prices making cards legal at those times:
commons $0.10 or less, uncommons $0.20 or less, rares $0.40 or less and mythics $1.00 or less. The prices are checked by the secondary-market-trader "mtgotraders" and are enshrined by a Static Legal Card List. Other Heirloom formats base legality on being both Heirloom legal and legal for their particular format. (such as Standard, Extended, Highlander ect)
For more Details see Heirloom Rules.
Join the Community on MTGO in the Heirloom chatroom for questions and matches: