Paging down or up a subfile
To make the Subfile page to the next or previous screen you should set a name to the Subfile indicator that tells you whether there is another page to show.
This is usually a plus sign ("+") but may vary.
If the field is present we can assume there is another page:
if ( CHECK_FIELD_EXISTS("SFL_MORE") )
{
SENDKEY(KeyPageDown);
}