relation between sets. either "union" or "interset".
Value
A numeric vector or NULL if no file id is well-defined.
Details
File-set is deined by case or file category. files belonging to a
case/filecategory are in a set. This function get file id from the
selected sets. When multiple sets are selected, the relation between
them can be define. When relation is union, file ids from either
selected set are returned. When reltion is intersect, only file ids
appear in all the selected sets are returned.