What does INTERNAL form properties mean? On the Drupal form-API page, there is this paragraph that reads:


  Note that internal properties are not included in the table below, but
  are included in the Properties list. These include: #built, #children,
   #error, #id, #input, #printed, #validation_arguments


What does "internal" mean, as compared to other form element property types?
Internal means that these properties are not supposed to be used by module developer.