Sorry I was a bit vague.
I meant the data associated with a particular audio file. I had been working on a recording done over Skype, and which was saved as an mp3 file. When I worked on it in WaveLab, all I saw in the directory was the mp3 file itself.
The documentation on file management is, like much of the WaveLab 7 documentation, rather inadequate. After reading some more and experimenting, I discover that I can store “companion files” alongside (in the same directory as) the audio file. The WaveLab 7 Elements manual does not even mention “companion files”, so I have no idea what they might contain.
As a consequence of the lack of documentation I have very little sense of how files are handled. Is editing non-destructive across separate WaveLab sessions? If so, then why, when I save a file at the end of a session, is the file (apparently) re-encoded?