See https://chat.fhir.org/#narrow/stream/197730-Da-Vinci.20PDex.20Drug.20Formulary/topic/PlanID.20Extension

  The description of PlanID is HIOS-centric.   Needs to be generalized.
 
Is the intention to have a unique plan ID for government mandated plans or all plans (including commercial)?
If the latter, could do something simple like: Unique, Payor generated Plan ID number. (Plan IDs must be unique, even across different markets.)
If the former, could do something like: Unique, generated Plan ID number, such as HIOS ID for QHPs or Contract Number for Medicare Advantage Plans. (Plan IDs must be unique, even across different markets.)