Publisher Item Identifier (PII) is a numbering system used to provide unique citation reference for electronic publications. The PII system was created in 1996 and is now used by major science and engineering publishers such as American Chemical Society, the American Institute of Physics, the American Physical Society, Elsevier Science, and the IEEE.

A Wikipedia article explaining the PII numbering system can be found on: http://en.wikipedia.org/wiki/Publisher_Item_Identifier