Legendary Insight is the overall raid currency.It serves as a proof of experience for fractal challenge modes. Unstable Cosmic Essence was previously acquired from the completion of Shattered Observatory Challenge Mote.As most experienced groups want to kill the bosses as fast as possible, kill proof requirements are very common in raid parties.
Pinging kill proof is mostly done to avoid inexperienced players, or players that are new to a certain boss. Linking the kill proof in chat is done via pinging the items upon joining a party.
Kill proof (often abbreviated as KP) is an unofficial term used among players to prove they have killed a raid or Fractals of the Mists boss and are experienced with it.