The codes and materials are:
1 - Polyethylene Terephthalate (PET)
2 - High Density Polyethylene (HDPE)
3 - Polyvinyl Chloride (PVC)
4 - Low Density Polyethylene (LDPE)
5 - Polypropylene (PP)
6 - Polystyrene (PS)
7 - Other Plastics
The material in which it is made of actually determines how recycling may occur. For instance, many recycling operations will not accept Code 7 material, because the wide variety of plastics makes recycling difficult.
Plastics coded 1 and 2 are recycled the most (Ref 2). Other less recyclable plastics are polyvinyl chloride (PVC - code 3), low-density polyethylene (LDPE code 4), polypropylene (PP - code 5), polystyrene (PS - code 6) and any other plastic (code 7).