The order (effective in a few hours) is here. There is no general list of reasons, though in a specific case one might be able to find evidence as to why a certain item was restricted. However, you also won't get a very specific list, instead you'll get a description of the kinds of goods and services that can't be exported. For instance, included are "significant goods or services used in connection with the automotive sector of Iran", but that does not say whether cup holders are significant goods, though I imagine that transmissions are. Sanctions are also directed at individuals, but the document does not list all of the sanctioned individuals. This is such a list, which is not limited to Iran.