Private fields are fields that in a way are user-defined fields but there is no common standard to access their contents. Some of them are used to transport DRM.
MP3tag ignores these and does not even tell you that they are there.
For further information you may have a look at
http://id3.org/id3v2.3.0
Section 28: Private Frame http://id3.org/id3v2.3.0#Private_frame