it means it will replace a Common card in your pack (if there’s any), as in it will use that slot instead of denying you a card from superior rarity.
For example your next pack would be 1 Golden Leg, 2 rares, 3 commons. If by chance that pack is chosen to have a Signature, it would occupy one of the 3 commons, instead of replacing another superior rarity card so the new Signatures are always a favorable addition to the game for the player.
it means it will replace a Common card in your pack (if there’s any), as in it will use that slot instead of denying you a card from superior rarity.
For example your next pack would be 1 Golden Leg, 2 rares, 3 commons. If by chance that pack is chosen to have a Signature, it would occupy one of the 3 commons, instead of replacing another superior rarity card so the new Signatures are always a favorable addition to the game for the player.