A, the contract, that is, the contract, needs to stipulate the transaction quantity, amount, payment method, transaction method, port of departure, port of destination, method, etc.
B, PI is the packing list, which needs to list the product name, parts, weight, ruler, date, shipping marks and other information.
C, PI are proforma invoices, which indicate the amount, unit price, number of pieces, etc.
D and CI are commercial invoices. Some companies want commercial invoices before delivery, while others don't. Our factory communicates directly with foreigners, never doing CI, but having PI is ok.
You have almost understood it here, but it is far from enough. If you want to export, you need to know the insurance policy, certificate of origin, electronic products and related certification materials, etc., which is more troublesome.