2.1.3 Variable Length Properties
A variable length property, within the context of this document, is defined as one where each instance of the property can have a value of a different size. Such properties are specified along with their lengths or have alternate mechanisms (such as terminating null characters) for determining their size.
Following is an exhaustive list of property types that are either variable length or stored in a stream like variable length property types. These property types are specified in [MS-OXCDATA] section 2.11.1.
Each variable length property has an entry in the property stream. However, the entry contains only the property tag, a flag providing more information about the property, the size, and a reserved field. The entry does not contain the variable length property's value. Since the value can be variable in length, it is stored in an individual stream by itself. Properties of type PtypGuid do not have variable length values (they are always 16 bytes long). However, like variable length properties, they are stored in a stream by themselves in the .msg file because the values have a large size. Therefore, they are grouped along with variable length properties.
The name of the stream where the value of a particular variable length property is stored is determined by its property tag. The stream name is created by prefixing a string containing the hexadecimal representation of the property tag with the string "__substg1.0_". For example, if the property is PidTagSubject ([MS-OXPROPS] section 2.1034), the name of the stream is "__substg1.0_0037001F", where "0037001F" is the hexadecimal representation of the property tag for PidTagSubject.
If the PidTagStoreSupportMask property (section 2.1.1.1) is present and has the STORE_UNICODE_OK (bitmask 0x00040000) flag set, all string properties in the .msg file MUST be present in Unicode format. If the PidTagStoreSupportMask is not available in the property stream or if the STORE_UNICODE_OK flag is not set, the .msg file is considered to be non-Unicode and all string properties in the file MUST be in non-Unicode format.
All string properties for a Message object MUST be either Unicode or non-Unicode. The .msg File Format does not allow the presence of both simultaneously.