files fall into three main categories or "content types". Files outside these categories are not of interest: Content is considered to be "Mixed content" if files fall into more than one of the three categories. Content is considered to be Unknown if the content sniffing process finds no categorisable content. This does not mean that there are no files present, just no files falling into one or more of the above three content types. In this case AutoPlay opens an Explorer window on the root directory of the medium and the process flow finishes here. PerceivedType In order to class files into the three content types, the PerceivedType value associated with various file extensions is used to make the category determination. PerceivedType can be set to "image", "audio" or "video" to class files into Pictures, Music or Video content types respectively. There are other PerceivedTypes known to Windows (text, compressed, system and application) but these are not relevant to AutoPlay. All file type information is stored in the Registry under HKEY_CLASSES_ROOT. The following is the PerceivedType Registry information for .avi Video files and .mp3 Music files: HKEY_CLASSES_ROOT\.avi (Default) "avifile" PerceivedType "video" HKEY_CLASSES_ROOT\.mp3 (Default) "mp3file" PerceivedType "audio" EventHandler (volume) The EventHandler is the title of a table that makes an association between various Handlers and the triggering Event. In the case of volume-based events, the EventHandlers are generated completely internally to AutoPlay with no reference to the Registry. Note that examination of the section of the Windows Registry reveals the and keys. Off these keys, various subkeys and values would appear to relate to and even control volume-based AutoPlay. However, as of Windows XP SP2, these keys are defined solely for future use and are not accessed by AutoPlay. One of the following predefined EventHandlers are chosen based solely on the decision AutoPlay makes internally after volume examination and content sniffing: Non-volumes Device characteristics As Windows cannot perform content sniffing on non-volume devices, AutoPlay analyses these devices by their characteristics rather than by their content. Every newly discovered device has identification information, normally stored within the device itself. This information is gather by enumerators and is used to build identification strings which Windows refers to when accessing and referring to the device. An identification string for a particular device is called a Device ID. In order to uniquely identify each instance of each device that is installed, Windows assigns an Instance