求各位前辈看下，这个客户是什么意思？

内容如下： 这是他的第一封邮件: qty: 500 pcs. as frame contract; 5 call-offs with 100 pcs. (total qty will be taken within 12 months). delivery immediately after call-off, payment after the delivery (no advance payment for the frame contract). delivery with 5 pcs. on one ippc pallet (120*800mm) delivery term: ddu 42489 wuelfrath, germany tecnical drawing attached 下一封邮件： what i meant with my previous mail is, that we would need a price for the following (if it is acceptable for you): you produce 500 pcs total (frame contract), but the shipment every couple of weeks will only be about 100 pcs. (5 shipments), so that the total quantity will be delivered in about 6-8 months. if it is not convenient for you to keeping the qty on stock, you could maybe quote only 100 pcs and we will place new orders every couple of weeks. 这里面的call-off 是啥意思？ 是不是这种协议签了，就是虽然是叫我们生产500套，但是每次他只拿100套，分五次，但是如果中间有问题的话，可能就会取消订单，换下单其他类型的产品？？那我们工厂岂不是要有库存留下了哇？？ 求各位前辈指导！新人不怎么懂。。。