The "Subfiles" page of the Restraints tab is only available if you are currently using an ICDD PDF database as reference database. Here you can select the PDF subfiles from which entries are accepted.
By default, PDF entries of all subfiles are accepted. You can change this simply by marking the checkboxes of the subfiles you would like to accept. If you would like to select only a few or nearly all subfiles, you can use the Clear All or the Select All buttons at the bottom of the tabsheet.