Hi Divya,
I'm not sure how the report is laid out and what the requirement is, however if you don't link the Subreport with a field, the Subreport will run like a separate report and might return all records (if you don't have a selection formula).
Since this is a Subreport, we usually link it with a database field (in this case it could be the Customer ID or another field) so that the Subreport returns data 'relative' to the Main Report, for e.g you might have a group on Customer ID. So each Subreport should return data for that specific group and hence the link.
-Abhilash