The resin identification code, (also known simply as a plastic's "recycling codear ") is a number inside the universal symbol for recycling (three turning, mutually chasing arrows; ♲) found on a plastic informing you of its polymer type. Developed by the Society of the Plastics Industry at the urging of recyclers in 1988, it allows easy identification of a plastic item. At the time of writing, there are presently eight different codes:
6: PS - polystyrene (packaging peanuts)
7: O or OTHER - fiberglass, acrylic, nylon, Nalgene bottles, multilayer plastics and others
Source: http://plasticsindustry.org