I am reading a paper that uses the terms "Boundary Fragment Class", "Boundary Fragment Structure" and "Boundary Fragment" without first defining it. When I google these terms, I see papers that use the terms without defining them and I can't just get a link to a direct definition. Can someone please explain what these mean? For example, what does it mean for a function/classifier to have "boundary fragment structure"? This is from a Machine Learning paper and they were referring to functions so I think Math is the correct site for this question.