I think people have two different things in mind with using the words "mild case of CF"
First there is the clinical term which is based on Pulmonary Function Test results (PFT) FEV1 results which go something like this: (pulling this out of my head - I'm sure I'm off a little)
over 85% normal lung function
76% - 85% is mild lung disease
40% - 75% is moderate lung disease
below 40% is severe lung disease
So in terms of having "mild CF" if your FEV1 is in the mild category - you are "a mild case" Which we all know can and will likely change over time and sometimes changes quite rapidly.
Then, I think that parent's and doctors alike sometime use the words "mild case" <b>more loosely to mean </b>something along the lines of "not as severe as we have seen others at this age" or "not as many difficult symptoms to deal with as others"
There is no denying that people with CF have very different case histories -- some with multiple hospitalizations for very life threatening episode, some with just a few hospitalizations for tune ups only, and still others, like my kids with no hospitalizations and very few symptoms at all (so far) and many variations in between.
There is always much debate on using the words mild and CF together, but that's my take on it. I am aware of how quickly things can and do change, but generally speaking I feel I can honestly say my kids are more mild than others -- <i>it doesn't mean anything </i>-- just gives a little perspective or something to compare to when trying to explain the differences in people with CF.
**edited to add a third thing
Doctors sometimes use the term "mild variant" when talking about genes that are class 4 & 5 because those genes are sometimes associated with more mild symptoms and/or CAVD in men. Again, I'm not saying it means anything conclusively in terms of what to expect, just that some genes fall into that category and doctors and sometimes parents use the term loosely to describe.