According to the ICAO, the first 2 digits are for the type of document, the codes for the document type are:
P: Passport
V: Visa
A: Identity Card (exact use defined by the Issuing State)
C: Identity Card (exact use defined by the Issuing State)
I: Identity Card (exact use defined by the Issuing State)
AC: Crew Member Certificate
IP: Passport Card
When you just need one digit to identify a document you use the < . An example of when the 2nd digit is used is for a diplomatic passport (Not sure, but maybe PD)