the subreport can be anything you want it to be. However if you want dynamic content as a link, then you need the content (linked field) to be in the main report.
I did not understand your first question re: header and footer...?
If you are linking the main report to the subreport, the value that is passed in the link is dependent on where you place the sub report. In the report header it will pass the value from the first row in the main report, in the report footer it will pass the last value in the main report.