Because the genre is so undefined it is truly impossible to say. I think it has a lot to do with the background of the person performing it - if they regularly perform in folk clubs, or used to, their songs are more likely to be thought of as 'folk'. On the other hand you get people writing acoustic songs which might be considered 'folk' who strenuously resist the label because 'folk' is usually unfashionable and commercial death.

Often it's a label of convenience for anything vaguely acoustic which doesn't conveniently fit into the charts' straitjacket.