ID3v2 HEADER OVERVIEW
The first part of the ID3v2 tag is the 10 byte tag header, laid out as follows:
ID3v2/file identifier "ID3" ID3v2 version $04 00 ID3v2 flags %abcd0000 ID3v2 size 4 * %0xxxxxxx
The first three bytes of the tag are always "ID3", to indicate that this is an ID3v2 tag, directly followed by the two version bytes. The first byte of ID3v2 version is its major version, while the second byte is its revision number. In this case this is ID3v2.4.0. All revisions are backwards compatible while major versions are not. If software with ID3v2.4.0 and below support should encounter version five or higher it should simply ignore the whole tag.
MEANING OF FLAGS
ID3v2 flags %abcd0000
The version is followed by the ID3v2 flags field, of which currently four flags are used:
a - Unsynchronisation
Bit 7 in the 'ID3v2 flags' indicates whether or not unsynchronisation is applied on all frames (see section 6.1 for details); a set bit indicates usage.
b - Extended header
The second bit (bit 6) indicates whether or not the header is followed by an extended header. The extended header is described in section 3.2. A set bit indicates the presence of an extended header.
c - Experimental indicator
The third bit (bit 5) is used as an 'experimental indicator'. This flag SHALL always be set when the tag is in an experimental stage.
d - Footer present
Bit 4 indicates that a footer (section 3.4) is present at the very end of the tag. A set bit indicates the presence of a footer.
All the other flags MUST be cleared. If one of these undefined flags are set, the tag might not be readable for a parser that does not know the flags function.
The ID3v2 tag size is stored as a 32 bit synchsafe integer (section 6.2), making a total of 28 effective bits (representing up to 256MB).
The ID3v2 tag size is the sum of the byte length of the extended header, the padding and the frames after unsynchronisation. If a footer is present this equals to ('total size' - 20) bytes, otherwise ('total size' - 10) bytes.