What is WMA ? 

Both WMA and WMV extensions uses ASF structure to store audio and video data including additional information about them. In fact there is no WMA extension that is only ASF file.

The ASF specification is available at Windows Media. You can read this specification to find out every thing about WMA and WMV files.

ASF made of objects. In fact ASF is object that contains sub objects. Each object have it own structure. As an example there is an object contains file properties and another contain audio data. Every objects have a GUID as identifier. ASF files starts with special GUID that uses as ASF file indicator. GUID is 128 bits length. Every objects starts with GUID and follow by size of object. The data part of object have it own structure.

There is an object named "Content Description Object" the table below is structure of this object:

Field name	Field type	Size (bits)
Object ID	GUID	128
Object Size	QWORD	64
Title Length	WORD	16
Author Length	WORD	16
Copyright Length	WORD	16
Description Length	WORD	16
Rating Length	WORD	16
Title	WCHAR	Varies
Author	WCHAR	Varies
Copyright	WCHAR	Varies
Description	WCHAR	Varies
Rating	WCHAR	Varies

As you can see in the table, ASF structure used 16bit number for length of strings. This technique used every time that ASF needs to store strings (Differ from string storage in ID3 tagging). And everywhere ASF use unicode encoding for strings (In ID3 string encoding is custom).

As it's clear "Content Description Object" is not enough to store file information. There is too many thing such as picture that's not supported in "Content Description Object". There is another object to store other information. In fact "Content Description Object" is for basic information of file and "Extended Content Description Object" used for more descriptions. This is the structure of this object:

Field name	Field type	Size (bits)
Object ID	GUID	128
Object Size	QWORD	64
Content Descriptors Count	WORD	16
Field Name Field Type Size (bits) Descriptor Name Length WORD 16 Descriptor Name WCHAR varies Descriptor Value Data Type WORD 16 Descriptor Value Length WORD 16 Descriptor Value See text varies

As any other objects this object starts with ObjectID and Size. Indicate number of Descriptors that object contains and then for each descriptor have Descriptor structure. Descriptors can contains Strings, Digits, Binary data or any other types.

Professional Tag Editor supports "Content Description", "Extended Content Description", "Branding Object". To see how Professional Tag Editor reads and write ASF information take a look at How it works (WMA).