The Retired Reserve includes "all reserve members who receive retirement pay on the basis of their active duty and/or reserve service," as well as "those members who are otherwise eligible for retirement pay but have not reached age 60 and who have not elected discharge and are not voluntary members of the Ready Reserve or Standby Reserve."
Citation: Office of the Chairman of the Joint Chiefs of Staff. (2019). Dictionary of Military and Associated Terms. Washington DC: The Joint Staff.