这段话是怎么解释？

A device need not have additional control endpoints besides endpoint 0 (although the USB specification allows for the possibility) because endpoint 0 can service most control requests perfectly well. If you define a vendor-specific request that can\'t complete within the frame, however, you should create an additional control endpoint to forestall having your onboard handler preempted by a new transaction.
他解释了什么时候要自己建立0号端点，但是我还是不明白他解释的原因。

对，开始我写错了呵呵。主要是这句话我不太明白：
如果你定义的厂商请求在一侦内无法完成，那么需要定义额外的控制端点！