Larissa,
Just think of it like so many other ESRI quirks that are not documented. I think if your source data has the fields but are null, then you will be able to map them. I have been working with it for the past few weeks, and found that I had forgot one field and couldn't do an append until I added the field (even though it had a null). I agree that it should be documented somewhere, but I was unable to find it.