ID3 tag field name = compilation

The fields are actually called Frames in MP3 ID3. ID3v2 contains "Advanced ID3v2 info" frames which basically allow custom frames to be defined, as opposed to ID3v1 where you?re stuck with ONLY the native frames (frames predefined to that version of ID3). Because the new and custom frames are defined in their own block of space, no problems are caused with backwards compatibility. As more custom frames are developed, ID3 may add them to their list of native frames in the next version of ID3, hence ID3v2.1, ID3v2.2, etc. Once these frames become native, they are identified by assigned 4 letter codes (Eg. PCNT (Play counter), TBPM (beats per minute), and TCOM (Composer)). For more information about the above, click here (http://id3lib.sourceforge.net/id3/id3v2.4.0-frames.txt).

That having been said, it appears that the compilation frame isn?t yet implemented in ID3 up to the most recent ID3v2.4. Instead, iTunes uses an additional custom frame to register if an MP3 is part of a compilation. It's titled "custom text frame" and it's value is "1", making the total size for that frame "3" (bytes).