The format currently allows the entry of up to two subseries, which are entered into fields #501 and #502. These fields behave in exactly the same way as #500, and may have the same subfields.
#500 小說界文庫|9xiao shuo jie wen ku
#501 長篇小說系列|9chang pian xiao shuo xi lie
ISBD: (小說界文庫. 長篇小說系列)
#500 皇冠叢書|n1834|9huang guan cong shu
#501 三毛全集|n23|9san mao quan ji
ISBD: (皇冠叢書 ; 1834. 三毛全集 ; 23)
However, it is very rare for a series to have more than one subseries; there are currently only 21 in the catalogue (20 April 2016):
#500 中國田野考古報告集|9zhong guo tian ye kao gu bao gao ji
#501 考古學專刊. 丁種|n42|9kao gu xue zhuan kan. ding zhong
#502 黃河水庫考古報告|n5|9huang he shui ku kao gu bao gao
ISBD: (中國田野考古報告集. 考古學專刊. 丁種 ; 42. 黃河水庫考古報告 ; 5)