The Nights of Reckoning expansion contained so-called rules cards. Each NoR booster also contained one rules cards (from a set of five rules cards) describing the new rules for the set. For added use in draft or sealed play, each rules card also doubled as a library card as indicated on the card when turned upside down.
Initially these cards were not allowed as part of decks in constructed tournaments. But after some discussion in the VtES Usenet Newsgroup, a month after the initial release of Nights of Reckoning (on May 10th, 2006), the rules cards became legal for all V:EKN tournaments, both constructed and limited.
Reference: Nights of Reckoning rules cards.