Yes, it does seem as though we're defining a "tri-valued" state differently. To me any of the three values (Yes, No or Null) can be set by the user, at any time. As well as re-set by the user. By this I mean that a field can start as Null, changed by the user to Yes, and then changed back to Null.
While everyone is saying this is a Yes/No field, it really depends on how the user decides to implement it. Here are some possibilities that I can imagine:
Purchase paper edition? Yes, No, Null = decision not yet made
Whose ebook is this? Yes = Dad, No = Mom, Null = Kid (yeah, multi-text would be better, but someone will do something similar)
Ebook's metadata requires updating? Yes, No, Null = unknown
I'm sure that with a bit more thought I could come up with other examples. But I'm at work and it's taken me six hours to get this far!
