外贸合同翻译付款方式

3000Tpershipment.50%downpaymentand50T/Tbeforeshipment.PaidbyCNY.Thesellerexhangesthecurrency,andthebuyershouldpayfortheexchangecost.
备注这段我没看懂....是下批货款必须在前批发货后15天内付款么?
Thepaymentofnextshipmentshouldbepaidwithin15daysfromthedateoflastshipment.