Remember the movie, Do the Right Thing? That's pretty much what a purchasing code of conduct is for a company trying to be socially responsible. The code is a statement of ethical principles and appropriate practices that the company adheres to, and expects its suppliers and sub-contractors to adhere to as well. Socially responsible codes of conduct for purchasing often cover things like worker safety, environmental quality, child labor standards, and so on.
Need more information? Newspaper archives are one of the best online research tools.