Hi,
You are correct that the Doc begin date, Doc end date, start date and stop dates are all taken from the index data that goes with the file. For an AFP file, it would be embedded in tags in the file, and for generic indexed docs it would come from within the .ind file.
In both cases, the date field that is used is defined in the Application Group and the Application that you have defined. For example, you have a field called "MyDate" (type Date) in the Application Group, which you have tagged as "Segment" on the Field Information tab, using the corresponding check box. And you have also defined in the Application (for example) that it should be picked from page 1 of every new "Group" (aka each independent doc in the input file), row X column Y, and Z characters long. And/or you have defined the date Format to match the date in your input documents, so it gets interpreted correctly, of course.
The highest and lowest date(s) found inside the loaded file is used to determine these dates in the System Load log, and you can see it in the MsgNum 87 in the System Log too.
Hope this helps!