A hierarchy of classes used for storing metadata about each of your classes. A ProjectData object contains PackageData objects which contain ClassData objects which contain LineData objects. Or, in bad ASCII art:

ProjectData <--> PackageData <--> ClassData <--> LineData

The SourceFileData class is not used during instrumention or testing. It is a only used when generating the reports. Basically it's used to lump a set of classes together when those classes come from the same file. This is needed because the HTML reporting displays an entire file at a time, instead of displaying a single class at a time.