In the Parcel Packing > Postage Options tab you can enter the default info about your shipments (31). This info will be used in customs declarations as a description of the content of your shipments. You can override this info for each order.
The tariff number is required by some carriers for international shipments. You can find your tariff number here and, if you sell different products in your store, you should set the correct tariff number on each product.