If a series is organized into sub-series, then all material within the series must be filed in a sub-series.
Sub-series may be determined by genre, format, or subject matter. A series of Writings, for example, can be organized into sub-series of Books, Plays, and Shorter Works. A series of Photographs can be organized into sub-series of Albums, Portraits, Group Shots, People, and Places.
When arranging at the sub-series level, it is not necessary to examine the contents of all files in order to determine appropriate sub-series. A targeted examination of key files, or a less thorough examination of a range of files may provide the necessary information.