Component subtype
A four-character code that identifies the type of the media handler or data handler.
Overview
For media handlers, this field defines the type of data — for example, 'vide' for video data, 'soun' for sound data or ‘subt’ for subtitles. See Media data atom types for information about defined media data types.
For data handlers, this field defines the data reference type; for example, a component subtype value of 'alis' identifies a file alias.