SHOULD use x-extensible-enum

enum values SHOULD use the marker x-extensible-enum rather than enum.

Invalid Example

deliveryMethods:
  type: string
  enum:
    - PARCEL
    - LETTER
    - EMAIL

Valid Example

deliveryMethods:
  type: string
  x-extensible-enum:
    - PARCEL
    - LETTER
    - EMAIL

Zalando Guideline 112