Custom data processors 

Implementing a custom data processor is out of scope in this reference. You can find information on the implementation in TYPO3 Explained.

Custom data processors can be used in TypoScript just like any other data processor:

tt_content {
  myextension_dataproccustom =< lib.contentElement
  myextension_dataproccustom {
    templateName = DataProcCustom
    dataProcessing.10 = custom-category
    dataProcessing.10 {
      as = categories
    }
  }
}

The available configuration depends on the implementation of the specific custom data processor.