USG CLASSIFICATION AND SECRECY

The starting point for any discussion of US Government (USG) classification & secrecy is CAPCO:

http://www.danjryan.com/jscrpt.html
http://www.fas.org/sgp/library/moynihan/foreword.html
http://www.gpo.gov/congress/commissions/secrecy/index.

Prior to CAPCO, USG classification was haphazard and inconsistant. DoD, DoS and IC (and DoE as well) each had their own methods and markings, which were largely incompatible.
CAPCO standardized the processes and the markings.

CAPCO is managed by the National Archive, Information Security Oversight Office (ISOO). ISOO produces manuals and guidances on CAPCO, such as ISOO Implementing Directive No. 1
See http://www.archives.gov/isoo/training/marking-booklet.pdf
ISOO Dir No. 1 is the starting point for understanding.

Overarching DoD implementation of this can be found in DoD 5200.1
http://www.fas.org/irp/doddir/dod/5200-1ph/index.html

A more thorough discussion can be found in the NIMA guide (NIMA has since renamed to NGA):
http://ftp.fas.org/sgp/othergov/dod/nimaguide.pdf
Of particualar interest is introduction of the / separators. This document has extensive tables of possible combinations.
Also notes appropriate use of FOUO (and improper use)
Later section go in to great detail about using the full codes.