“Partner” means “You may designate two legendary cards as your commander rather than one if each of them has partner.”
October 13, 2023 · Doctor Who
“Partner” means “You may designate two legendary creature cards as your commander rather than one if each of them has partner.”
June 10, 2022 · Commander Legends: Battle for Baldur’s Gate
“Choose a Background” is a variant of the partner ability that represents a static ability. “Choose a Background” means “You may designate two cards as your commander as long as one of them is this card and the other is a legendary Background enchantment.” You can’t designate two cards as your commander if one has a “choose a Background” ability and the other is not a Background enchantment, and legendary Background enchantments cannot be your commander unless you have also designated a commander with “choose a Background.”