Jess Garcia, director of trading card games at Heritage Auctions, sees the forces driving it clearly
702.139c: Once you take the special action and put the card with companion into your hand, it remains in the game until the game ends
Although Pokmon and Magic: The Gathering are clear leaders when it comes to the genre of trading card games, many other well-known games have emerged, including KeyForge, Yu-Gi-Oh!, Game of Thrones: The Card Game, and Sorcerer
These packs come directly from an ex-Wizards of the Coast (WOTC) employee and were forgotten about for 26 years before being brought to us
If it has an ability that triggers "when you unlock this door," it will trigger when the Room enters
Play Boosters have Special Guests cards available in non-foil, while Collector Boosters have them in traditional foil