This article is from the Paper Money Collecting FAQ, by Bruce Giese firstname.lastname@example.org with numerous contributions by others.
Refers to Albert Pick, author of the world paper money book
on general issues. Nearly everyone who deals in world paper
uses this system for identifying notes. Every note within a
country has a number associated with it, often shown in the
form "P-34" for general issues and "P-S34" for specialized
issues (volume 1). Since every country has the same numbers,
a note is identified by country and Pick number. For example,
USA P-480 is your garden variety series 1988 1 dollar bill.