The letters 'Pam' at the start of a shelfmark indicate that the item in question is a pamphlet, which could get lost or damaged on standard book shelves. In the Main library, pamphlets are kept in boxes on Level 1 in the West Wing. At Palace Green, pamphlets are housed in the same room as standard books of the same classmark.
For more information:
http://www.dur.ac.uk/resources/library/ ... COLOUR.pdf