In this dialog you can specify the relations between a subreport and the main report. Using this relations you can filter the records in the subreport or customize the subreport depending on it's value.

Using double click on the main report field or drag and drop (left to right) you can add a main report field to the subreport link list. Using a double click on the subreport link you can remove it from the list. When you select an added field you can choose a field of the subreport which will be linked to this field.

The prompt field which is used for this connection will be displayed also. If you need a more complex link you can edit the record selection formula of the subreport.