There is a fourth tier named Platinum and requires 150,000 tier miles. There is an additional membership known as Invitation Only or "IO" where you must be invited to this small group of members. The benefits provided are over and above the Platinum benefits.
A higher-tier designation called Platinum requires 150,000 tier miles. There’s also a special membership known as Invitation Only (IO), where invitations are needed to join this small group of members. These benefits go beyond what is provided by the Platinum membership.