Each country has its own laws and regulations for the import and export of goods, which is applied by customs authorithy. In most countries, customs are subordonated to government agreements and international laws, the import and export of some goods may be restricted or forbidden. Therefore there is a tax on the important or exportation (more rarely) of goods called a customs duty. Until processed through custom clearance are held in a custom area and for the record, all authorized ports are recognized customs areas.
In China, customs clearance can be conducted at a custom office. For the shipment, a commercial invoice, packing list and CIF (cost, insurance, freight) must be provided. Also, on all documentation should stand the complete name of the country "People's Republic of China" and not only "China".
For any products requiring Chinese certificates, copies of all the documents must be enclosed to the exporting goods (CCC, CEL, CFDA, CRCC…). Concerning assembled products that are certified as a system, the goods must be send assembled to China otherwise the customs officers may ask certificates for every single components. Many companies expect a smooth customs clearance process when importing in China, because their products are not mandatory for a certification. However, many products in this case are held in customs clearance area with a note saying "certifications required". In fact, most of the time the HS code, product name or product specification implies a certification obligation. Finally, if products are wrongfully detained in customs (products not obliged for certification), the manufacturer still has to cover the costs of storage, or destroy the goods, or send them back to the country of origin.