A work may belong to more than one series, and each may have subseries. The format accommodates up to 3 series, each of which may have up to 2 subseries. The possibility that more may be needed is remote, and the Oxford database currently contains very few works that even belong to more than one series (20 April 2016).
#500 福建檔案資料叢書|9fu jian dang an zi liao cong shu
#510 廣東檔案資料叢刊|9guang dong dang an zi liao cong kan
ISBD: (福建檔案資料叢書)(廣東檔案資料叢刊)
It is often difficult to be certain that what appears to be a second series is not, in fact, a subseries of the first, and vice versa:
#500 隋唐歷史文化叢書|9sui tang li shi wen hua cong shu
#510 人文叢刊|n10|9ren wen cong kan
ISBD: (隋唐歷史文化叢書)(人文叢刊 ; 10)
There are currently only 27 examples of works that belong to three different series (20 April 2016), and only 3 where a second series also has a subseries, and in all cases the distinction between series and subseries is far from certain:
#500 江蘇文史資料|n94|9jiang su wen shi zi liao
#510 武進文史資料|n18|9wu xi wen shi zi liao
#520 武進人物叢書|n2|9wu jin ren wu cong shu
ISBD: (江蘇文史資料 ; 94) (武進文史資料 ; 18) (武進人物叢書 ; 2)
#500 寧蒗縣納西族家庭婚姻調查|n3|9ning lang xian na xi zu jia ting hun yin diao cha #510 國家民委民族問題五種叢書|9guo jia min wei min zu wen ti wu zhong cong shu #511 中國少數民族社會歷史調查資料叢刊|9zhong guo shao shu min zu she hui li shi diao cha zi liao cong kan
ISBD: (寧蒗縣納西族家庭婚姻調查 ; 3) (國家民委民族問題五種叢書. 中國少數民族社會歷史調查資料叢刊)