Relation p3p#purpose (p3p#Statement,p3p#PurposeClass)  Defines the purpose for which the data practise is employed
  supertype:  pm#P3P_relation (pm)
  subtype:  p3p#purposeAlways__purposealway (p3p#Statement,p3p#PurposeClass)  The purpose is always required; users cannot opt-in or opt-out of this use of their data.
  subtype:  p3p#purposeOptIn (p3p#Statement,p3p#PurposeClass)  Data may be used for this purpose only when the user affirmatively requests this use.
  subtype:  p3p#purposeOptOut (p3p#Statement,p3p#PurposeClass)  Data may be used for this purpose unless the user requests that it not be used in this way.