Consider reviewing the file msidefs.h from platform SDK's include directory for more insights. In this case, you'll want to check the enum msidbCustomActionType for the first ten values.

// CustomAction.Type
enum msidbCustomActionType
{
 // executable types
 msidbCustomActionTypeDll              = 0x00000001,  // Target = entry point name
 msidbCustomActionTypeExe              = 0x00000002,  // Target = command line args
 msidbCustomActionTypeTextData         = 0x00000003,  // Target = text string to be formatted and set into property
 msidbCustomActionTypeJScript          = 0x00000005,  // Target = entry point name, null if none to call
 msidbCustomActionTypeVBScript         = 0x00000006,  // Target = entry point name, null if none to call
 msidbCustomActionTypeInstall          = 0x00000007,  // Target = property list for nested engine initialization

 // source of code
 msidbCustomActionTypeBinaryData       = 0x00000000,  // Source = Binary.Name, data stored in stream
 msidbCustomActionTypeSourceFile       = 0x00000010,  // Source = File.File, file part of installation
 msidbCustomActionTypeDirectory        = 0x00000020,  // Source = Directory.Directory, folder containing existing file
 msidbCustomActionTypeProperty         = 0x00000030,  // Source = Property.Property, full path to executable

 ...

}

The MSDN topic above covers the valid combinations of these bit flags.

Robert Flaming
Windows Installer Program Manager
Team Blog: http://blogs.msdn.com/windows_installer_team
Individual Blog: http://blogs.msdn.com/rflaming