1.3.1 Compound Files
The .msg File Format is based on the Compound File Binary File Format, which is described in [MS-CFB]. The paradigm provides for the concept of storages and streams (1), which are similar to directories and files, except that the entire hierarchy of storages and streams are packaged into a single file, called a compound file. This facility allows applications to store complex, structured data in a single file. For more information regarding structured storage in a compound file, see [MSDN-STS].
The format specifies a number of storages, each representing one major component of the Message object. A number of streams (1) are contained within those storages, each stream representing a property (or a set of properties) of that component. Nesting is possible, as described by [MS-CFB], where one storage can contain substorages.