Centrality in event generator is defined using impact parameter of collision b.
The range of impact parameter used in MC simultaion is defined in dataset name (and subdirectory name) like '/b0_3/','/b0_12/' etc. where b0_3 means that impact parameter b has been taken in the range 0-3 fm.

Centrality	Notation in the directory	Range of impact parameter	Estimated percentage of HIJING events (from Ron)
minbias	/b0_10/ up to /b0_20/	0-10 fm, 0-20 fm
central	/b0_3/ up to /b0_6/	0-3 fm, 0-6 fm	~20% for b=0-6 fm
midcentral	/b3_6/	3-6 fm	~13%
midperipheral	/b6_9/	6-9 fm	~30%
peripheral	/b9_12/, /b8_15/	9-12 fm, 8-15 fm	~40% for b=9-12 fm

Lidia Didenko